fix: reaper meta key for android, evidence-walk cost, atomic writes

[ElbertePlinio]

Closes #46.

P0

• packages/android/src/session.ts: create/destroy now set/clear REAPER_CLEANUP_PENDING_META_KEY using the same protocol as desktop-linux/browser (set on a failed stop so the emulator stays trackable, clear once cleanup is confirmed complete). Added packages/android/test/reaper.test.ts, which forces a destroy failure and proves reapDeadRunningSessions still finds and cleans up the leaked emulator.

P1 — performance & reliability

• packages/core/src/evidence.ts: appendAction no longer does a full recursive readdir+lstat walk of the run directory on every action. The artifact byte total is cached per run dir and only rescanned when a cheap entry-name snapshot shows the counted tree actually changed (filtered the same way the walk is, so journal-lock/truncation-sentinel churn never forces a rescan). The journal's own live size comes off the already-open handle. Orphaned on-disk artifacts (never referenced by any journaled action) are still caught, matching prior behavior — covered by a new cache/perf test plus the existing cumulative-bytes tests, all passing.
• packages/mcp-server/src/resources.ts: run-log reads now cap memory use like the screenshot guard at :528 — an oversized log is read via tail semantics (last ~1MB off an open handle) instead of loading the whole file before redacting secrets.
• packages/browser/src/supervisor.ts: the child-shim's stderr buffer is now bounded to the last 64KB when no newline arrives, instead of growing unbounded.

P1 — simplify

• Hoisted writeFileAtomic (tmp + rename, mode-preserving, cleans up the tmp file on any failure) from agent-installers/src/atomicFile.ts into packages/core/src/paths.ts (next to ensureDir), exported from core's index. Used in core/session.ts writeSession, run.ts writeManifest, config.ts writeConfigFile (all three previously leaked the tmp file on rename failure), agent-installers' own call sites, and the two already-safe evidence.ts call sites (active-pointer publish, truncation-sentinel commit) — evidence.ts's concurrency-critical claim/lock logic is otherwise untouched.

Validation

• bun run typecheck (tsc --noEmit) clean across the workspace.
• Full vitest run (root bun run test equivalent): 78 files, 950 passed / 2 skipped, including packages/core, packages/android, packages/browser, packages/mcp-server.
• No sudo usage introduced; MCP surfaces untouched w.r.t. privilege.
• No overlap with Architecture: deepen local runtime seams #44's lifecycle-centralization work — this only fixes the android reaper protocol and hoists a shared write helper.