Cut 0.4.0

[Seungpyo1007]

Cuts 0.4.0.

The release was planned as a codebase index. It shipped without one, and the measurements behind that decision are the most reusable part of it: symbol lookup through the language tooling already running answers in 150–280 ms, returned the true definition first in every query measured, and produced 19 results where a text search produced 105 for the same name. Building a second index would have had to beat that while staying correct as files change on disk. docs/PLAN-0.4.md keeps the numbers and is marked consumed.

What the release contains instead:

• Finding a definition — symbol search works in TypeScript projects for the first time (Ctrl+T had only ever asked the language server, and TypeScript is served by Monaco's own worker); find_symbol and find_references for the agent; import lines are no longer offered as definitions; tests rank below source; an exact name is never truncated away by partial matches from earlier files.
• Saying "I don't know" — five places that reported an unknown as an absence: a large TypeScript project answered as not being TypeScript, a missing language server answered as an empty symbol list, a model that cannot make tool calls appeared to work, a failed reference lookup answered as no references, and watcher overflow was lost silently.
• Extensions — an extension can own a file type (registerCustomEditorProvider), and the debug namespace exposes sessions and breakpoints. Binary custom editors and registerDebugAdapterDescriptorFactory remain unimplemented and now fail loudly; both are documented in the README.

Version bumped to 0.4.0, CHANGELOG written.

1133 unit tests pass; typecheck and build clean.