fix(tui): show completed write output

[kitlangton]

What

Show the syntax-highlighted file contents after the V2 write tool completes.

This is a port of #41352, which was accidentally merged into the stale v2-migration branch and never reached v2. Same change, cherry-picked onto v2.

Before / After

Before: The write renderer used metadata.diagnostics as its completion signal. V2 writes do not emit diagnostics yet, so successful writes remained as a compact Write <path> row and hid the content already present in input.content.

After: A completed write switches to the existing code-block presentation based on the durable tool status. Diagnostics remain optional and render when available.

How

• packages/tui/src/routes/session/index.tsx keys the write block on state.status === "completed" instead of optional diagnostics metadata.

Scope

• This does not add V2 LSP diagnostics to write or edit tools.
• Branched from c4bea9d558 rather than current v2 HEAD: c217ebe2ad has pre-existing repo-wide typecheck failures in packages/plugin (missing SystemPart/CommandInfo/ConnectionInfo exports) that block the pre-push hook and are unrelated to this change.

Testing

• bun run test test/mini/tool.test.ts test/mini/entry.body.test.ts test/mini/scrollback.surface.test.ts test/cli/tui/inline-tool-wrap-snapshot.test.tsx in packages/tui (50 passed)
• packages/tui typecheck passes at the branch base; current v2 HEAD has unrelated pre-existing failures noted above