GeezerVison

Sets up the foundation for a comprehensive unit-test campaign on the
socket-patch CLI. No behavior change to the binary.

Two changes:

1. Expose the clap parser as a library so integration tests can verify
   the public CLI contract without spawning the binary:
   - new crates/socket-patch-cli/src/lib.rs hosting Cli, Commands,
     looks_like_uuid, and parse_with_uuid_fallback (extracted from main.rs)
   - Cargo.toml gains [lib] entry alongside the existing [[bin]]
   - main.rs is now a thin wrapper that delegates to the lib

2. De-duplicate the manifest-path resolution block that was copy-pasted
   into 5 commands (apply, rollback, list, remove, repair). New helper
   socket_patch_core::manifest::operations::resolve_manifest_path
   handles the absolute-vs-relative join in one place, with 3 unit tests.

3. New CLI_CONTRACT.md next to the crate documenting every subcommand,
   flag, default, alias, JSON shape, and exit code as semver-significant
   surface. Adds a comment block above pub enum Commands pointing to it
   so anyone editing the parser sees the contract reminder.

Verified: cargo build/clippy/test --workspace --all-features all clean
(415 unit tests pass, including 3 new resolve_manifest_path tests).

Foundation for follow-up PRs that add the per-command parser snapshot
tests and helper unit tests.

Assisted-by: Claude Code:claude-opus-4-7

Add a parser-level integration test that locks in the public clap contract
for the `repair` subcommand. The key assertion is that `--download-mode`
defaults to `"file"` — diverging from every other command's `"diff"`
default. `repair` restores legacy per-file blobs needed to apply any
patch, so a silent flip to `"diff"` would break that path with no
parser-level signal. The `gc` visible alias is also exercised (both bare
and with a flag) so a refactor that drops it is caught immediately.
Covers every flag in the repair row of CLI_CONTRACT.md plus an
unknown-flag failure case that pins the clap error kind.

Assisted-by: Claude Code:claude-opus-4-7