Use git rev-parse --git-dir so startup preflight follows worktree .git indirections to the real metadata directory, then check directory permission metadata without creating probe files. Add a regression that verifies both the warning kind and structured event path for a read-only external gitdir.
Constraint: ROADMAP #695 requires early startup/worktree diagnostics without destructive writes or broad sandbox redesign.
Rejected: write-probe detection | it mutates git metadata during a diagnostic path.
Confidence: high
Scope-risk: narrow
Directive: Keep startup preflight warnings non-destructive and structured by warning kind/path.
Tested: cargo fmt --manifest-path rust/Cargo.toml --all -- --check; cargo test --manifest-path rust/Cargo.toml -p runtime startup_preflight -- --nocapture; cargo test --manifest-path rust/Cargo.toml -p runtime worker_boot -- --nocapture; cargo check --manifest-path rust/Cargo.toml --workspace
Not-tested: full cargo test --manifest-path rust/Cargo.toml --workspace
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