feat(voyage)!: rename produced_by field values + validator comments [skip-docs]

- commands/trekexecute.md: produced_by literals -> trekexecute (4 occurrences)
- commands/trekendsession.md: produced_by literals -> trekendsession (2 occurrences)
- tests/validators/next-session-prompt-validator.test.mjs: 11 'ultraexecute-local' refs -> 'trekexecute'
- tests/commands/trekcontinue.test.mjs: 3 fixture strings updated
- tests/lib/cleanup.test.mjs: 1 fixture string updated
- lib/validators/next-session-prompt-validator.mjs: producer-list comment
- docs/HANDOVER-CONTRACTS.md line 432: example producer names updated

Part of voyage-rebrand session 2 (W3.4 / Step 6).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/ultraplan-local/docs/HANDOVER-CONTRACTS.md

@@ -429,7 +429,7 @@ The state file follows a producer/consumer separation that keeps responsibilitie
 
 **Frontmatter contract for `NEXT-SESSION-PROMPT.local.md`:** Producers MUST write a YAML frontmatter block on the prompt file with at minimum:
 
-- `produced_by:` — string identifying the producer (e.g. `ultraplan-local-A4-session`, `ultraexecute-local-phase-8`, `ultraplan-end-session-local`)
+- `produced_by:` — string identifying the producer (e.g. `trekplan-A4-session`, `trekexecute-phase-8`, `trekendsession`)
 - `produced_at:` — ISO-8601 timestamp of when the file was written
 
 The `next-session-prompt-validator` (`lib/validators/next-session-prompt-validator.mjs`) cross-checks `produced_at` against the sibling state file's `updated_at` and emits a `NEXT_SESSION_PROMPT_INCONSISTENT` error when the prompt is older than the state — that means the prompt has not been refreshed for the current session and is stale. Files **without** any frontmatter are tolerated (warning, not error) for backwards compatibility with v3.3.x and earlier hand-rolled prompt files; this is consistent with Handover 3's drift-WARN posture.