Phase 1: Separate CLI-only lockfile code from runtime public API

[FL4TLiN3]

Parent

Part of #504 — Make @perstack/runtime isomorphic

Problem

findLockfile() and loadLockfile() are exported from @perstack/runtime's public API but use Node.js-specific APIs:

• readFileSync from node:fs
• path.dirname, path.resolve, path.parse, path.join from node:path
• process.cwd()

These functions are only consumed by @perstack/tui (start-handler, run-handler).

Current state

• RunOptions.lockfile already exists — callers can pass a pre-resolved Lockfile object
• findLockfile/loadLockfile are convenience helpers that resolve the lockfile from the filesystem

Goal

Remove findLockfile/loadLockfile from runtime's main export so that the core runtime package has no node:fs/node:path dependency for lockfile resolution. Determine the appropriate new home for these helpers (TUI layer, new utility package, or runtime sub-path export).

Files involved

• apps/runtime/src/helpers/lockfile.ts
• apps/runtime/src/index.ts
• packages/tui/src/start-handler.ts
• packages/tui/src/run-handler.ts

[FL4TLiN3]

Completed in #510. findLockfile/loadLockfile moved from @perstack/runtime to @perstack/tui (lockfile.ts). Runtime no longer has node:fs/node:path dependency for lockfile resolution.