feat(boatstack): retro derive — recurring instructions become typed proposals

[bigboateng]

Final slice of the published → merged program (follows #242). This closes the loop the whole program serves: the system can now detect the next standing prompt on its own and propose the typed control that retires it.

What changed

• Gap classifier (internal/retromine/classify.go): a deterministic keyword lexicon places each recurring instruction into missing_observation / missing_verb / missing_setpoint / missing_guard, with fixed precedence guard > setpoint > observation > verb (a guard misread as a verb could become an action proposal, so the constraining readings win). Anything unmatched lands in unclassified — surfaced, but generating zero proposals (fail-closed).
• Report (report.go): typed proposals with occurrences, sessions, capped exemplar, evidence refs, and a SuggestedShape — prose naming the promotion ("a persisted goal like delivery.terminal…"), never a diff.
• retro derive CLI: boatstack-helper retro derive --input <transcript> ... [--format events|claudecode|plaintext] [--json]. The CLI boundary owns the pipeline's only I/O — reading the operator-named files; Boatstack never scans for transcripts. Output goes to stdout only.
• "retro" joins the coverage-exemption set beside "flow" with its rationale pinned (derive-only, no delivery transition).
• SKILL.md's retro operation gains the concrete step: derive → review → promote by hand through the reviewed flow; never a saved prompt, never auto-applied.

Boundary conformance

• Boundary: transcript-derived analysis meeting the repository's change authority.
• Control law: retro-proposes-never-enforces (the same law as lab 21's interlock derive: derivation proposes, promotion is a reviewed human change).
• Required evidence: retro_conformance_test.go — positive (each gap type classifies from planted recurring phrasing with a suggested shape; rendering states the never-enforces contract), negative (unclassified recurrence → zero proposals), bypass (derivation leaves the filesystem byte-identical and never modifies its inputs), failure-state (empty input → empty report exit 0; malformed transcript → typed error, never a partial report).
• Failure behavior: fail-closed to unclassified; parse failures are typed and positional.
• Release condition: full suite green; synthetic fixtures only.

Evidence

• go build ./... && go vet ./... && go test ./... — pass (all four packages).
• End-to-end: fed a synthetic transcript of the exact instruction that motivated this program — "open the PR, watch CI until every check passes, then merge it" — the CLI reports [missing_setpoint] … Promote it: Add a typed setpoint (like delivery.terminal). The miner independently prescribes the control the program built.
• Release note added: 2026-07-28-retro-derive-proposals.md.