Add test coverage for the resume rollout-fork detector

[Juliusolsson05]

The resume rollout-fork detector landed in #9 without unit tests (the agent-code repo convention is no new test files in feature/fix PRs). It is the most logic-dense and most testable part of that change.

Surface to cover (src/CodexHeadless.ts, src/proxy/CodexResponsesAdapter.ts):

• collectRolloutLineageIds — id extraction, session_meta exclusion, cap.
• normalizeCwd — symlink / case / .. / trailing-slash canonicalisation.
• isResumeForkCandidate — mtime guard, cwd match, lineage-overlap threshold, empty-lineage fail-closed, same-cwd-no-lineage rejection.
• watchResumeForkRollout — a fork created before, during, and after attach is all evaluated (the priming-race regression fix(codex): correct rollout tail ownership so the committed feed keeps growing #9 fixed).
• CodexResponsesAdapter — a 'completed' flow drops late chunks and is reaped.

Some of these may need light extraction to be test-addressable. isResumeForkCandidate and the lineage helpers are close to pure already.

Refs #9, Juliusolsson05/agent-code#159.