feat: redirect git invocations to jj inside jj repos

fuckgit installs as a Cargo bin literally named `git` (via
`[[bin]] name = "git"`). When invoked inside a jj worktree by an
AI agent, it prints the equivalent `jj` subcommand plus the live
`jj help <sub>` output to stderr and exits 2 — the AI sees a clear
failure and the right correction. Outside a jj repo, or with no
intercept marker, it is a transparent passthrough.

The intercept gate fires only when:
- effective cwd (after honoring `git -C`) is inside a `.jj` worktree
- FUCKGIT_INTERCEPT=1 or an AI marker (CLAUDECODE) is set
- subcommand is not on the plumbing allowlist
  (rev-parse, cat-file, for-each-ref, ...)
- GIT_DIR / GIT_EXEC_PATH / CARGO are not set
- no top-level --git-dir= / --work-tree= flag is present

Otherwise we walk PATH, skip self by canonicalized current_exe(),
and exec() the first other `git` we find — preserving signals,
exit code, and TTY ownership.

Escape hatches: FUCKGIT_PASSTHROUGH=1 env var, --fucking-git flag
(stripped before exec). FUCKGIT_DEPTH guard aborts at depth >= 2
to prevent self-recursion.

Linux and macOS, zero runtime dependencies.

.gitignore

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+/target