feat(miner-hands): compose an immutable per-attempt acceptance-criteria document - #4449

Closes #4271

What

Before a coding-agent driver (#4262's interface; #4266/#4267's implementations) starts editing, the miner needs to pin down — immutably, so the agent can't quietly redefine its own success bar mid-attempt — what "done" means for this attempt. This adds the pure composition step that folds the two already-shipped Phase 2 primitives into a single acceptance-criteria document:

• the sanitized PromptPacket (prompt-packet.ts:26, the analyze→coding-agent "boundary membrane"), and
• the FeasibilityGateResult go/raise/avoid verdict (feasibility.ts:21-26).

This is the direct input to preparePlanTemplate's coding-agent step (plan-templates.ts:84, which runs after branch-create and before local-test). Producing the document is this module's job; writing it into the attempt worktree (#4269) and handing it to the driver interface (#4262) are separate concerns, so this stays pure and side-effect-free like the rest of gittensory-engine.

Decisions (the issue's open questions, resolved here)

• Serialization: JSON, not markdown. The criteria are an immutable, checksum-verifiable success bar consumed by tooling (a self-review step must be able to prove the target didn't move across iterations), so a deterministic canonically-ordered JSON document beats prose. serializeAcceptanceCriteria emits stable-key-order JSON with a trailing newline so a recorded checksum stays byte-stable across processes.
• Filename: one fixed name, ACCEPTANCE_CRITERIA_FILENAME (acceptance-criteria.json).
• Immutability as a real property: the built document is deep-frozen (Object.freeze, arrays copied+frozen) so it can't be mutated in-memory for the lifetime of the attempt; the byte-stable serialization is what a caller checksums on disk.
• Written only on go: a raise/avoid verdict means the attempt should not start, so no criteria file is written. buildAcceptanceCriteria still returns a document with writable: false so a caller can log why it was skipped; shouldWriteAcceptanceCriteria is the single gate for the write itself.

Redaction is delegated to sanitizePromptPacketField rather than re-implemented, since this document is exactly as exposed to a prompt-injectable coding-agent session as the prompt packet already is (idempotent on already-sanitized text).

API

• buildAcceptanceCriteria({ promptPacket, feasibility }) => AcceptanceCriteria — pure, deep-frozen.
• serializeAcceptanceCriteria(doc) => string — deterministic canonical JSON + trailing newline.
• shouldWriteAcceptanceCriteria(verdict) => boolean — the go-only write gate.
• ACCEPTANCE_CRITERIA_FILENAME, ACCEPTANCE_CRITERIA_VERSION.

Files

• packages/gittensory-engine/src/miner/acceptance-criteria.ts — the composer.
• packages/gittensory-engine/src/index.ts — barrel re-export.
• test/unit/miner-acceptance-criteria.test.ts — tests (composition from both inputs, redaction pass-through, raise/avoid non-writable cases, deep-freeze/immutability, deterministic serialization).

Testing

npx vitest run test/unit/miner-acceptance-criteria.test.ts
npm run typecheck

8/8 tests pass; typecheck clean. New engine file is fully covered (7/7 lines, 3/3 functions, no uncovered branches).