feat: behavior coverage, pre-mortem levels, Status elimination, disk-evidence selection

[nullhack]

Summary

• Behavioral Distinctness + Behavior Pre-Mortem: Adds 4th pre-mortem level (behavior) with per-rule-type prompts. Adds Behavioral Distinctness concept to gherkin knowledge — partition examples by Then outcome, not input value. Structural/invariant rules get one representative example; full coverage deferred to Hypothesis property tests.
• Invariant Property Tests: Widens property test scope from @bug only to all invariant/structural rules. Adds Invariant Property Tests concept to test-design knowledge.
• Status field elimination: Removes Status: ELICITING | BASELINED from feature files entirely. Status is derivable from content (no file = not discovered, description only = discovery, rules but no @ids = breakdown, @id-tagged examples = baselined). Replaces feature-status: ==BASELINED flow condition with baseline-confirmed: ==verified.
• Disk-evidence feature selection: Rewrites select-feature skill to detect delivery status from disk evidence (diff @id tags vs test functions + test run results) instead of reading Status fields.
• Template rename: feature.feature.template → <feature_name>.feature.template (consistent with <type_id> placeholder convention in AGENTS.md).

Files changed

Knowledge (5 files)

• gherkin.md — Behavioral Distinctness concept + common mistake + key takeaway
• pre-mortem.md — 4th level Behavior Pre-Mortem with prompts for action/behavioural/structural rules
• test-design.md — Invariant Property Tests concept, widened property test table scope
• feature-discovery.md — content-based lifecycle instead of Status enum
• wsjf.md — @id-tagged Examples eligibility instead of Status: BASELINED

Skills (4 files)

• write-bdd-features/SKILL.md — step 5: group by Then, discard duplicates, behavior pre-mortem
• write-test/SKILL.md — step 1: structural rules also get Hypothesis property tests
• select-feature/SKILL.md — rewrite: detect delivery status from disk evidence
• discover-features/SKILL.md — remove Status: ELICITING from file creation
• confirm-baseline/SKILL.md — remove redundant Status check

Flow (1 file)

• planning-flow.yaml — feature-status: ==BASELINED → baseline-confirmed: ==verified

Template (1 file)

• feature.feature.template → <feature_name>.feature.template, Status line removed

AGENTS.md — template path example updated