feat(tool): add interactive terminal tool with persistent PTY sessions

[herjarsa]

Summary

Closes #23759

Adds a fully interactive terminal tool that lets the agent manage persistent PTY sessions - create, send input, read output (with cursor-based incremental reads), and close sessions. This makes the terminal a true extension of the agent rather than a one-shot command runner.

What's included

• 5 action handlers: run, create, send, read, close
• SessionState: tracks exitCode (via sentinel command on Windows), discriminated union Zod schema
• FIFO eviction: max 20 sessions, oldest evicted first
• InstanceState: per-directory session isolation via Pty.Service
• Incremental reads: cursor-based read action returns only new output since last read
• Windows support: sentinel command pattern for exit code detection on PowerShell

Files changed

File	Lines	Purpose
src/tool/terminal.ts	640	Tool implementation with 5 actions, session state, eviction
src/tool/terminal.txt	112	Tool description for agent context
src/tool/txt.d.ts	4	Type declaration for .txt imports
src/tool/registry.ts	+7	Registration: import, service deps, init, builtin list
test/tool/terminal.test.ts	273	20 unit + 3 integration tests (23 pass, 2 skip)
package.json	+1	@babel/types devDep (Windows Bun workaround)

Testing

• 23 tests pass (20 unit + 3 integration)
• 2 skipped (backward-compat run action tests - bus event propagation limitation in test context)

Design decisions

• Sentinel command over Pty.onExit: Windows PowerShell does not reliably emit exit events, so we inject a sentinel command that writes EXIT_CODE:<N> to stdout and parse it
• FIFO eviction: simple and predictable - when max sessions reached, the oldest is closed and evicted
• InstanceState: sessions are scoped per working directory, preventing cross-project contamination
• Incremental reads: cursor-based approach avoids re-reading entire buffer, keeps context lean

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