fix: defer built-in tool overrides until session lifecycle

[RaviTharuma]

Summary

• Built-in tool overrides (read/grep/find/ls/bash/edit/write) were still registered at extension load time.
• That races with packages that also own bash (notably pi-patty-bg-tasks) and produces a hard Pi startup diagnostic:

  Error: Failed to load extension ".../pi-patty-bg-tasks/index.ts": Tool "bash" conflicts with .../pi-tool-display/index.ts
  

• README already claimed deferred ownership discovery (Built-in tool override ownership check is load-order-dependent, causing hard crash when another extension owns read/edit/etc. #29 / ownership docs), but the implementation still called registerIfOwned(...) immediately during load.
• This PR queues built-in overrides at load and only registers them from session_start / before_agent_start after re-checking pi.getAllTools(). If a non-builtin owner already claimed the tool, pi-tool-display yields.

Local workaround (already documented)

{
  "registerToolOverrides": {
    "bash": false
  }
}

Test plan

• npx tsx --test tests/tool-overrides-registration.test.ts
• Updated reload tests for deferred registration (tests/reload-behavior.test.ts)
• New regression: late external bash ownership (pi-patty-bg-tasks load order)
• With both packages enabled and registerToolOverrides.bash: true, Pi starts without a tool conflict diagnostic once this lands

Fixes coexistence with pi-patty-bg-tasks without requiring package reordering.

[RaviTharuma]

Tracked by #35 (repro with pi-patty-bg-tasks). Companion note on the other package: patty-io/pi-patty-bg-tasks#3