fix(config): update() writes to existing config file instead of hardcoding config.json

[dibstern]

Summary

• Config.update() hardcoded config.json as the write target, ignoring the project's actual config file. Projects using opencode.jsonc got a spurious config.json created alongside their real config, and changes (e.g. persisted permission rules via PATCH /config) were effectively lost since config.json is not loaded as project config.
• Adds localConfigFile() to resolve the existing project config file (opencode.jsonc > opencode.json), mirroring the existing globalConfigFile() pattern. Handles .jsonc files with patchJsonc() to preserve comments, matching updateGlobal().

Reproduction

1. Create a project with opencode.jsonc as the config file
2. Use PATCH /config (e.g. via "Always Allow" on a permission prompt)
3. Observe: a new config.json is created instead of opencode.jsonc being updated
4. The permission rule is silently lost on next startup since config.json is not a valid project config filename

Changes

• packages/opencode/src/config/config.ts: Added localConfigFile() resolver; rewrote update() to use it with .jsonc support
• packages/opencode/test/config/config.test.ts: Updated existing test; added new test verifying opencode.jsonc is updated and config.json is not created

Verification

• All 66 config tests pass (bun test test/config/config.test.ts)
• Typecheck passes across all 13 packages (pre-push hook)

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a potentially related PR:

PR #12347: fix: PATCH /config writes to correct project config file

This PR appears to address the same issue - fixing PATCH /config to write to the correct project config file instead of hardcoding config.json. It may already contain a solution to this problem or be related to the same underlying bug. You should check if this PR was already merged or if PR #16979 supersedes it with additional improvements.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.