feat: default to blocks tool display + native edit diff viewer

[justin-carper]

Summary

Make "blocks" the default tool display and render Cursor file edits with opencode's native diff viewer.

Cursor's internal tool activity previously defaulted to compact "reasoning" lines. This switches the default to "blocks" (structured, provider-executed tool parts) and, for edits specifically, lights up opencode's built-in diff viewer.

Changes

Default → "blocks"

• cursorEventsToStream / cursorEventsToContent default to "blocks"; createCursor sets toolDisplay: options.toolDisplay ?? "blocks".
• "reasoning" remains the documented fallback for older / non-V3 hosts (opencode < 1.16). README + CHANGELOG updated.

Native diff viewer for edits

• Cursor edit activity is emitted under opencode's registered edit tool so its built-in diff viewer renders; the real unified diff (Cursor's result.value.diffString) is passed via metadata.diff.
• opencode's edit schema requires oldString/newString, which Cursor does not expose — these are reconstructed from the diff's -/+ lines purely to satisfy schema validation. The call is provider-executed, so they are never applied to disk.
• The edit tool-call is buffered until its result (the diff only arrives with the result), then emitted call→result.
• Any edit without a usable diff (errors, unexpected shapes, or a host without a registered edit tool) falls back to a safe cursor_edit block. All other tools remain cursor_* prefixed blocks.

Test infrastructure

• tsconfig.json now covers test/; a new tsconfig.build.json handles the src-only declaration emit (wired into tsup). This surfaced and fixed 10 stale-type test bugs the previously-excluded test/ dir had hidden.

Verification

• npm run typecheck, npm run build, and npm test (128 tests) all pass.
• Manually verified against opencode: Cursor edits render in the native diff viewer; non-edit tools render as cursor_* blocks.

Notes

• Reconstructed oldString/newString are faithful for single-hunk edits and approximate (concatenated) for multi-hunk; they appear in the transcript but are never executed.