[codex] preserve fsmonitor for worktree Git reads (#26880)

Codex forces `core.fsmonitor=false` on internal Git commands so a
repository cannot select an executable fsmonitor helper. This also
disables Git's built-in daemon for `status`, `diff`, and `ls-files`,
turning those worktree reads into full scans in large repositories.

Read the raw effective `core.fsmonitor` value and preserve it only when
Git interprets it as true and advertises built-in daemon support through
`git version --build-options`. Query uncommon boolean spellings back
through Git using the exact effective value. Unset, false, helper paths,
malformed values, probe failures, and unsupported Git builds continue to
force `core.fsmonitor=false`.

Centralize this policy in `git-utils` while keeping process execution in
the existing local and workspace-command adapters. Probe once per
worktree workflow and reuse the result for its Git commands, including
the TUI `/diff` path. Metadata-only commands and repository discovery
remain disabled without probing. Each probe and requested Git process
keeps its own existing timeout, and the decision is not cached because
layered and conditional Git configuration can change while Codex runs.

---------

Co-authored-by: Chris Bookholt <bookholt@openai.com>

codex-rs/git-utils/src/lib.rs

@@ -2,6 +2,7 @@ mod apply;
 mod baseline;
 mod branch;
 mod errors;
+mod fsmonitor;
 mod info;
 mod operations;
 mod platform;
@@ -21,6 +22,9 @@ pub use baseline::reset_git_repository;
 pub use branch::merge_base_with_head;
 pub use codex_protocol::protocol::GitSha;
 pub use errors::GitToolingError;
+pub use fsmonitor::FsmonitorOverride;
+pub use fsmonitor::FsmonitorProbeRunner;
+pub use fsmonitor::detect_fsmonitor_override;
 pub use info::CommitLogEntry;
 pub use info::GitDiffToRemote;
 pub use info::GitInfo;