Tracking S2: per-task observation registry (A2)

[easonLiangWorldedtech]

Tracking issue for the S2 / A2 PR (branch feat/observation-registry-s2; PR to be opened after review).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Item: S2 / A2 — Per-task observation registry. Stacked on #1383 (S1 version token); to be rebased onto main once #1383 lands.

Scope

• New src/core/task/observationRegistry.ts: per-task Map<absolutePath, { version, observedAt }>; the task owns the instance, so parent and subtask observations are independent.
• Task owns an instance; ReadFileTool records an observation after a successful read of an existing file (one extra fs.stat via computeVersionToken — the same call the S4 write guard will reuse; a stat failure leaves the target unobserved and does not fail the read).
• No behavior change — observations are recorded but not yet consulted (that is S4).

Acceptance

• Read path cost = +1 stat; no other observable change
• Registry unit tests (observe / get / replace-on-reobserve)
• ReadFileTool spec: a read of an existing file registers an observation with the on-disk version
• Subtask isolation: parent and subtask registries are independent
• CI green (ubuntu unit-test gate), 100% codecov patch coverage

Status

• PR: #1394 open (targets main; includes the S1 commits — merge after #1383)