docs: add git howto collection

Two reference files covering:
- git.md — config, daily workflow, diff, log, stash, remotes, tags
- git-branching.md — branches, merge, rebase, cherry-pick, bisect, reflog, reset, revert, worktrees

topics/git.md

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+# Git
+
+> Distributed version control — track, branch, merge, and collaborate.
+
+## Config
+
+```bash
+# Identity
+git config --global user.name "Your Name"
+git config --global user.email "you@example.com"
+
+# Editor
+git config --global core.editor vim
+
+# Default branch name
+git config --global init.defaultBranch main
+
+# Useful defaults
+git config --global pull.rebase true
+git config --global push.autoSetupRemote true
+git config --global rerere.enabled true
+git config --global diff.algorithm histogram
+git config --global merge.conflictstyle zdiff3
+
+# Aliases
+git config --global alias.st status
+git config --global alias.co checkout
+git config --global alias.br branch
+git config --global alias.lg "log --oneline --graph --all"
+
+# Show current config
+git config --list --show-origin
+```
+
+### Config Scope
+
+| Flag       | File                    | Purpose          |
+|------------|-------------------------|------------------|
+| `--system` | `/etc/gitconfig`        | All users        |
+| `--global` | `~/.gitconfig`          | Current user     |
+| `--local`  | `.git/config`           | Current repo     |
+
+Local overrides global overrides system.
+
+## Init and Clone
+
+```bash
+git init
+git init --bare repo.git               # server-side (no working tree)
+git clone https://github.com/user/repo.git
+git clone --depth 1 https://...        # shallow (latest commit only)
+git clone --branch v2.0 https://...    # specific branch/tag
+```
+
+## Daily Workflow
+
+```bash
+# Status
+git status
+git status -s                          # short format
+
+# Stage
+git add file.txt                       # specific file
+git add src/                           # directory
+git add -p                             # interactive hunk staging
+git add -u                             # stage modified/deleted, not new
+
+# Unstage
+git restore --staged file.txt
+git reset HEAD file.txt                # older syntax, same effect
+
+# Discard working tree changes
+git restore file.txt
+git checkout -- file.txt               # older syntax
+
+# Commit
+git commit -m "feat: add login page"
+git commit -am "fix: typo"            # stage tracked + commit
+git commit --amend                     # rewrite last commit
+git commit --amend --no-edit           # amend without changing message
+git commit --allow-empty -m "trigger"  # empty commit (CI triggers)
+```
+
+## Diff
+
+```bash
+# Working tree vs staging
+git diff
+
+# Staging vs last commit
+git diff --cached
+git diff --staged                      # same thing
+
+# Between commits
+git diff abc123..def456
+git diff HEAD~3..HEAD
+
+# Between branches
+git diff main..feature
+
+# Specific file
+git diff -- path/to/file.txt
+
+# Stats only
+git diff --stat
+git diff --name-only
+git diff --name-status                 # with A/M/D markers
+
+# Word-level diff
+git diff --word-diff
+```
+
+## Log
+
+```bash
+# Compact
+git log --oneline
+git log --oneline -10                  # last 10
+
+# Graph
+git log --oneline --graph --all
+
+# Detailed
+git log -p                             # with diffs
+git log --stat                         # with file stats
+
+# Filter
+git log --author="alice"
+git log --since="2025-01-01"
+git log --after="2 weeks ago"
+git log --grep="fix:"                  # search commit messages
+git log -S "function_name"            # search for string in diffs (pickaxe)
+git log -G "regex_pattern"            # search diffs with regex
+git log -- path/to/file               # commits touching file
+
+# Format
+git log --pretty=format:"%h %an %ar %s"
+git log --pretty=format:"%C(yellow)%h%Creset %s %C(dim)(%ar)%Creset"
+
+# Show single commit
+git show abc123
+git show HEAD~2:path/to/file          # file at specific commit
+```
+
+## Stash
+
+```bash
+# Save work in progress
+git stash
+git stash push -m "wip: api refactor"
+git stash push path/to/file            # stash specific files
+
+# List stashes
+git stash list
+
+# Restore
+git stash pop                          # apply + remove from stash
+git stash apply                        # apply, keep in stash
+git stash apply stash@{2}             # specific stash
+
+# Inspect
+git stash show                         # stat
+git stash show -p                      # full diff
+
+# Drop
+git stash drop stash@{0}
+git stash clear                        # drop all
+
+# Stash untracked files too
+git stash -u
+git stash --include-untracked
+```
+
+## Remotes
+
+```bash
+# List
+git remote -v
+
+# Add
+git remote add origin https://github.com/user/repo.git
+git remote add upstream https://github.com/original/repo.git
+
+# Change URL
+git remote set-url origin git@github.com:user/repo.git
+
+# Fetch
+git fetch                              # default remote
+git fetch --all                        # all remotes
+git fetch --prune                      # remove stale remote branches
+
+# Pull
+git pull                               # fetch + merge (or rebase if configured)
+git pull --rebase                      # fetch + rebase
+
+# Push
+git push
+git push -u origin main               # set upstream
+git push origin --delete feature-branch # delete remote branch
+git push --tags                        # push all tags
+git push origin v1.0                   # push specific tag
+```
+
+## Tags
+
+```bash
+# List
+git tag
+git tag -l "v1.*"
+
+# Create
+git tag v1.0                           # lightweight
+git tag -a v1.0 -m "Release 1.0"      # annotated (preferred)
+git tag -a v1.0 abc123                 # tag specific commit
+
+# Delete
+git tag -d v1.0
+git push origin --delete v1.0          # delete remote tag
+
+# Show
+git show v1.0
+```
+
+## .gitignore
+
+```gitignore
+# Patterns
+*.log                  # all .log files
+build/                 # directory
+!important.log         # exception (don't ignore this one)
+**/temp                # temp in any subdirectory
+doc/*.txt              # doc/notes.txt but not doc/sub/notes.txt
+```
+
+```bash
+# Ignore already-tracked file
+git rm --cached file.txt               # untrack without deleting
+echo "file.txt" >> .gitignore
+
+# Debug ignore rules
+git check-ignore -v path/to/file
+```
+
+## Gotchas
+
+- `git pull` without `--rebase` creates merge commits — set `pull.rebase true` globally
+- `git add .` stages everything including untracked — review with `git status` first
+- `--amend` rewrites history — never amend commits already pushed to shared branches
+- `git stash` does not stash untracked files unless `-u` is passed
+- `-S` (pickaxe) finds where a string was added/removed, not where it appears
+- `git log -- file` stops at renames — add `--follow` to track across renames
+- Tags aren't pushed by default — use `--tags` or push individually
+
+## See Also
+
+- `git-branching` — branches, merge, rebase, cherry-pick, recovery
+- [Pro Git book](https://git-scm.com/book/en/v2)
+- `git help <command>` — built-in manual pages