Implement from a typed issue: work from its acceptance as the contract, publish with intent = issue ref (lex-lang #949)

[alpibrupa]

Part of lex-lang epic alpibrusl/lex-lang#949 and the loom binding alpibrusl/lex-loom#521.

The manifesto's core move: the agent's interface is a contract to satisfy — "a type signature, a set of examples, a set of properties" — not a codebase to imitate. A typed issue is that contract. lex-code should:

1. Fetch the issue (by ref, from the local store or the hub) and read its declared acceptance — the API delta + examples (typed-delta), the failing example, or the metric predicate.
2. Generate to satisfy it: spec/build/test modes work from the acceptance as the spec, with the examples as the oracle the fix loop iterates against (they already gate publish).
3. Publish with intent = the issue ref (--intent-* already exists; carry the issue id in the intent so every op links back — issue ↔ intent ↔ ops ↔ attestation).
4. Propose acceptance for a free-form issue (the agent-refinement flow in lex-lang's "agent-refined acceptance" issue): given a shape-5 issue, propose signatures + examples / a failing example / a metric predicate for human approval.

The runner path in loom (#521) then becomes: role picks issue → lex-code implements from its contract → the gate closes it.

[alpibrupa]

Items 1–3 landed in #174: --issue=<id> renders the acceptance as the task, binds the session so writes publish with --intent-issue, adds the issue_show/issue_verify tools, and always ends with lex issue verify → [ISSUE_VERDICT]. Live-verified with local qwen3.8 on a typed_delta issue (verified; ops carry the issue-linked intent). Remaining: item 4 (agent-proposed acceptance for free_form issues, with human approval).