feat(engine): manifest set — two grammars, never mixed

[leeovery]

Stage 4 of the batching programme (design log: #504; base: #507). Lee's review catch on #507: the batch form's positional-first-pair-then-assigned-extras asymmetry.

Why

The mixed grammar was a compatibility artifact from bolting batching onto the original three-arg set. It reads wrong, and it's a misfire surface: a model must remember pair 1 is positional and pairs 2+ are assigned — backwards, it sets a field literally named task_map.x=y or feeds a field name as a value. No CLI convention mixes like this; uniform key=value is the standard multi-assign shape.

What

• Single field: set <path> <field> <value> — unchanged, everywhere.
• Batch: set <path> <field>=<value> [<field>=<value> …] — every pair identical, split on the first =, routed on = in the first field argument (field names never carry one, values with = still work in both forms).
• Mixed: refused loudly, naming both grammars, nothing written.
• Project set: single-form only — no caller ever batched it.

Every caller swept in the same change (engine and prose ship together — no compat window): nine prose sites (initialize-plan, both session-setups, plan-construction, define-phases, author-tasks, process-review-findings, write-tasks, initialize-specification), the legacy-split apply.cjs (caught by its own suite going red — the one programmatic caller), both test suites, USAGE/commands.md/CLAUDE.md.

Test plan

• New grammar suite: uniform batch happy path; positional single incl. a value containing =; mixed refused with nothing written; project extras refused.
• Full sweep validated by the suites that broke and re-greened: npm test 1556/1556, test:cli green, conventions lint 27/27, typecheck clean.

🤖 Generated with Claude Code

Stack

1. feat(engine): discovery-map add-batch — the harvest persists in one call (D7) #505
2. feat(engine): manifest apply — batched set/delete across one work unit (D7) #506
3. refactor(prose): task-map family folds onto existing batch forms (D7, stage 3) #507
4. feat(engine): manifest set — two grammars, never mixed #508 👈 current
5. refactor(prose): dump/read folds — subtree gets replace per-topic loops (D7, stage 5) #509
6. feat(engine): commit --plan — plan storage stages from recorded state (D7, stage 6) #510
7. fix: batching-stack review round — re-resolve rewired, guards symmetric, hardening pinned #512
8. fix: deep-dive round 2 — backbone rule flipped, pre-upgrade paths routed, flow bugs closed #513
9. fix(review-r3): engine hardening + crash-resume seams #514
10. feat(scoping): promotion routes through first-phase choice + bridge #515
11. fix(spec): pin the bugfix source name to the topic #516
12. fix(skills): gate-skip sentence accurate in gate-less phases #517
13. fix(review-entry): name the real upstream — specification and plan #518
14. fix(shared): strip triage-landing's unreachable superseded-research arm #519
15. feat(engine): refuse work-unit-level fields that shadow the phases tree #520
16. fix(discovery): first-written briefs join the full brief lifecycle #521
17. test(sim): pipeline simulation — engine driven end-to-end as a black box #522
18. fix(round-4): review findings — backfill keying, ratchet evasion, pull guard #523