fix(lsp): identify the target checkout in cross-worktree locations

[F0Rextasy]

What

I reviewed the full diff; definition results in sibling checkouts rendered identically to local files, so locations outside the session cwd now carry an explicit checkout suffix alongside the absolute path.

Why

Fixes #12569

Testing

• bun test packages/coding-agent/test/tools/lsp-regressions.test.ts -t cross-worktree (1 pass, 4 assertions: suffix present for sibling, absent for local, context reads the right file)
• bun --cwd=packages/coding-agent run check:types (pass)
• oxlint + oxfmt on all 3 changed files (pass)

---

• bun check passes
• Tested locally
• CHANGELOG updated with the required attribution (if user-facing; internal issue fixes use issue links, external contributions add the PR link and contributor credit after creation)

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