docs: test strategy document — classification, authoring guide, and CI pipeline map

[planetf1]

Summary

We have test/MARKERS_GUIDE.md as a marker reference, but there is no single document that explains the overall test strategy to a new contributor. Markers are just one aspect — we need a document that covers the full picture.

What's missing

A top-level test strategy document (TEST.md or docs/docs/guide/testing.md) that addresses:

• Philosophy — what we test, what we don't, and why (e.g. test contracts not implementations)
• Classification — how to decide between unit / integration / e2e / qualitative with concrete decision rules
• Authoring guide — how to write a good test: fixture discipline, mock discipline, assertion style, naming
• When tests run — the CI pipeline tiers (pre-commit, PR CI, nightly, pre-release) and what runs at each stage
• Local dev workflow — how to run subsets, interpret results, use -rs for skip reasons
• Examples — opt-in # pytest: comment system, when examples become tests
• Backend/resource gating — how predicates.py works, when to add a resource gate

Relationship to existing docs

• test/MARKERS_GUIDE.md is the marker reference — it may be renamed (e.g. test/TESTING_GUIDE.md or folded into the strategy doc) since markers are just one section of a broader guide
• AGENTS.md §3 and §10 are brief summaries that should point to this doc rather than duplicate it

Acceptance criteria

• Strategy document exists at an agreed path (discuss: TEST.md root vs docs/docs/guide/testing.md)
• Covers all sections listed above
• test/MARKERS_GUIDE.md either renamed/consolidated or cross-linked
• AGENTS.md §3/§10 updated to point to the new doc
• markdownlint clean (if placed under docs/docs/)

Parent

Part of #726 — Epic: Testing Infrastructure & Strategy Overhaul