docs(harness): add automated background testing sketchbook

[bakeb7j0]

Summary

Pre-/devspec working notes for Plan: Gamma — Wave-pattern test harness foundation (Epic #626 sub-issue #1). Captures the design conversation that produced the Plan's scope before the formal Dev Spec walk begins.

Changes

• Adds docs/automated-background-testing-sketchbook.md (360 lines).
• Records:
  • Why the harness exists (Plan Plan: Beta — Wave-pattern reliability + autonomy hardening (operational backlog campaign B) #607 net-zero-progress diagnosis).
  • Tier structure (cheap → expensive) and the Tier-1-then-Tier-4 sequencing decision.
  • Five locked decisions: repo (Wave-Engineering/ccwork-testtarget), runner (Python+pytest), GitHub fixtures (private under Wave-Engineering/), GitLab fixture (under gitlab.com/testtarget/), cost budget ($4–8/night).
  • Phase 1 / Phase 2 v0 scope (Tier 1 runtime + Tier 4 tests 4.1 / 4.6 / 4.8).
  • Tier 4 deferred test inventory (4.2–4.5 / 4.7).
  • Relationship to Epic Epic: Close the wave-pattern automated-testing gap (integration harness, daemon coverage, sub-agent contract enforcement) #626's other 8 sub-issues (independent Stories under Epic: Close the wave-pattern automated-testing gap (integration harness, daemon coverage, sub-agent contract enforcement) #626, not Plan: Gamma scope).
  • Four open questions deferred to /devspec walk.

Linked Issues

Closes #639

Test Plan

• File renders cleanly in GitHub markdown (tier table, headings, code blocks).
• Internal references to memory files (lesson_doc_singular_branch_prefix.md, lesson_destructive_test_homedir.md) name files that exist.
• Cross-reference to Epic Epic: Close the wave-pattern automated-testing gap (integration harness, daemon coverage, sub-agent contract enforcement) #626 resolves on the public GitHub issue tracker.
• No broken intra-document anchors.
• Validation suite (shellcheck, shfmt, frontmatter, regression tests) green.
• Code-reviewer findings addressed in-line (1 important + 3 minor folded into the same commit).
• Trivy scan: 0 HIGH/CRITICAL.

Notes

• 104 stale test failures exist on main (prior install.sh removal + discord-bot → disc-server migration left tests behind). Unrelated to this PR; flagged separately as the kind of rot Plan: Gamma's nightly run will surface continuously.
• Branch prefix: docs/ (plural) used because mcp__sdlc-server__ibm rejects doc/ (singular) despite the documented convention. Worth a follow-up to align the regex.