fix/deep review improvements (#36)

* chore: untrack local AI assistant dirs and coverage artifacts

Remove .opencode/, .codex/, and .gemini/ from version control and ignore
them going forward, along with coverage.out/coverage.html which were
already gitignored but coverage.out had previously been tracked in some
clones.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* docs: fix project name in first-time guide, document Go version

Replace stale "Entire" references with "Trace" in
docs/first-time-contributors.md (including the section anchor link) and
document the Go 1.26.4 toolchain requirement in CONTRIBUTING.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* docs(checkpoint): document MustCheckpointID constant-only contract

NewCheckpointID already provides the error-returning constructor and all
runtime call sites use it; MustCheckpointID has no non-test callers.
Document that it is reserved for constants and test fixtures so panics
cannot reach production paths.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* docs(explain): expand nolint:nilerr justifications with fallback behavior

Each suppressed nil-error return now documents why swallowing the error
is intentional and what the caller-visible fallback is (not-found
reporting instead of raw git/internal errors).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* ci: upload deadcode report and add hook bench target

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Co-authored-by: Race Tester <race@test.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: 3 people