docs(doubles): document bashunit::unmock and the double lifecycle #899

Context

bashunit::unmock is a public helper (src/test_doubles.sh:26) that removes a
mock or spy and cleans up its state files. It is missing from
docs/test-doubles.md — the page a user or agent reads to learn the doubles API.

Where it currently appears:

• docs/globals.md:101 — mentioned only in passing, in a note about the
  bashunit:: prefix
• src/runner.sh:2059 — the runner's own per-test cleanup
• tests/unit/coverage_core_test.sh:284 — one real usage in the suite

It is absent from docs/test-doubles.md, docs/public/bashunit-skill.md and
docs/ai-agents.md.

Proposal

Add a ## bashunit::unmock section to docs/test-doubles.md alongside mock and
spy, covering:

• signature bashunit::unmock "function" and what it removes (the function
  override plus the spy's times/params files)
• that it is a no-op for a name that was never mocked
• when you actually need it: the runner already unmocks after each test, so
  the case that matters is undoing a double established in
  set_up_before_script, or re-mocking the same command differently inside one
  test

That last point is the real documentation gap: without it a reader cannot tell
whether they are supposed to call unmock in every tear_down.

Why this matters for agentic coding

The per-test auto-cleanup is invisible from the docs, so an agent reading only
docs/test-doubles.md has two failure modes: adding defensive bashunit::unmock
calls to every tear_down (noise, and it reads as if leakage were a real risk),
or not knowing the helper exists when a set_up_before_script mock genuinely does
leak between tests. Stating the lifecycle once removes both.

Acceptance criteria

• bashunit::unmock documented in docs/test-doubles.md with an example
• Lifecycle stated explicitly: automatic after each test, manual for
  set_up_before_script doubles
• Added to docs/public/bashunit-skill.md and docs/ai-agents.md doubles
  sections
• make lint green