fix(spawn): capture tool args from tool_execution_start + journal filesTouched (#60)

[rz1989s]

Problem (#60 — #49 regression)

filesTouched (#49's structured partial-result) was empty on real edit-tool runs despite passing unit tests. The archived T1 run (2 edits + 13 bash calls, 17 files modified) reported "(none detected)", and the run:ended journal event had no filesTouched field at all.

Root cause (confirmed against the SDK)

The pi SDK's tool_execution_end event carries { toolCallId, toolName, result, isError } — no args. Only tool_execution_start carries args. The engine read e.args off the end event (always undefined on real runs):

• extractTouchedFiles got undefined → no paths → filesTouched empty;
• buildToolEvent stringified undefined → the journal's tool events showed args: "".
  The unit tests passed because their fake children put args on the end event — the exact "mocked shape ≠ SDK shape" dogfood pattern.

Changes

• spawnSubagent captures args per toolCallId at tool_execution_start and uses them on the end event (?? e.args fallback keeps hand-rolled fakes working).
• run:ended now journals filesTouched (previously SpawnResult-only — the durable journal a controller reads post-hoc lacked it). RunEndedEvent gains filesTouched?.

Tests

• Real SDK event pair (start with args, end without) on a completed run: filesTouched populated from the start event's args + journal tool event carries the real path (was "").
• Turn-budget cut: SpawnResult.filesTouched + run:ended.filesTouched both populated.
• 722/722 pass, pnpm typecheck clean.

Closes #60