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.

Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "fuckgit"
+version = "0.1.0"
+edition = "2024"
+rust-version = "1.88"
+authors = ["Stephen Waits <steve@waits.net>"]
+description = "A binary named `git` that redirects you to `jj` (Jujutsu) when invoked inside a jj repo."
+license = "MIT"
+readme = "README.md"
+repository = "https://github.com/swaits/fuckgit"
+homepage = "https://github.com/swaits/fuckgit"
+keywords = ["jj", "jujutsu", "git", "vcs"]
+categories = ["command-line-utilities", "development-tools"]
+
+[[bin]]
+name = "git"
+path = "src/main.rs"
+
+[dependencies]
+
+[profile.release]
+lto = "thin"
+strip = true
+codegen-units = 1