[split-1066] U12(1+2) - task save-stage semantics + finalize open partial tool ask #1933

Unit U12 of the PR #1066 split (1/6).

Split plan: #703
PR: #1927
Content source of record: tag pr1066-source = 46d1d218701f0ce2d675b1b315489bacb6b0f77d (easonLiangWorldedtech/Zoo-Code).

Unit contract

Task.saveClineMessages() reports the message-write stage independently of the metadata / task-history stage, and Task.finalizePartialToolAsk() closes an open partial tool ask by type + text.

Why this unit exists on its own

Single provider group (Task persistence) + single gate scope. Planned as two units (U1 stage semantics, U2 finalize) and merged because the stage-semantics tests call finalizePartialToolAsk() — the test blocks cannot be split without orphaning them (test-block atomicity).

Boundary

• base: 72143527fd33 (main / merge base of fix(write-to-file): address partial filesystem error review #1066)
• head: cf5abe64d6e8
• chain: (first) -> U12 -> U4
• merge order is fixed: U12 -> U4 -> U5 -> U3 -> U6 -> U7/FINAL; the FINAL integration PR (fix(write-to-file): clean partial state on missing-param and rooignore denial + integrate the #1066 split series (6/6 + FINAL) #1928) is the sole merge target.

Files and budget

• src/core/task/Task.ts +75/-5 — byte-identical to source
• src/core/task/__tests__/Task.spec.ts +400/-0 — byte-identical to source
• src/core/assistant-message/__tests__/presentAssistantMessage-custom-tool.spec.ts +1/-0 — allowNew: one mock line, no source change
• budget: 481 a+d / 3 files — SOFT-OVERSHOOT (rationale: single provider group + single gate scope; under the 1000 hard cap)
• mutation gate: 41 changed executable lines (extension package) — under the 500 cap; valid mutants for the whole PR are 116 / 400, so no directive was added by this unit.

Verification (must pass by once, binary)

• zdt split verify --contract U12.json --worktree <wt> --head cf5abe64d6e8 — PASS (every changed file is a content subset of the source of record, or an explicitly sanctioned allowNew file).
• Tests: 218 passed / 5 skipped, exit 0 (narrowest relevant suites: Task.spec.ts, writeToFileTool.spec.ts, writeToFileTool-partial-state-cleanup.spec.ts, removeClineFromStack-delegation.spec.ts, presentAssistantMessage-custom-tool.spec.ts).
• Changed-line coverage: 18 covered / 0 uncovered — PASS.
• ESLint --prune-suppressions --max-warnings=0: clean on every touched file; suppression counts unchanged.
• No .changeset file, no CHANGELOG edit (AGENTS.md).

Deviations recorded

• U1 + U2 merged into this unit (test-block atomicity).

Reproduce

git fetch https://github.com/easonLiangWorldedtech/Zoo-Code p1066/u1-task-save-stages-and-partial-ask
node zdt.mjs split verify --contract U12.json --worktree <wt> --head cf5abe64d6e8
node zdt.mjs split measure --worktree <wt> --base 72143527fd33 --head cf5abe64d6e8
pnpm --dir src exec eslint --prune-suppressions --max-warnings=0 <touched file>