fix(app): MEMORY / OOM crash - add experimental flag to disable vcs diff auto-fetch

[jeremyakers]

Issue for this PR

Partially fixes #24049 and #22683

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds an experimental disable_vcs_diff config flag and uses it to skip the web app's automatic vcs.diff fetch.

This is a mitigation for workspaces where /vcs/diff can be very expensive or crash the server, especially umbrella workspaces with nested git repos/worktrees.

This does not fully fix the underlying /vcs/diff scalability issue in #24049. It preserves the current default behavior and gives affected users an opt-in escape hatch while broader fixes are considered.

How did you verify your code works?

Tested locally with a built opencode binary against a large umbrella workspace.

• With experimental.disable_vcs_diff unset, the existing behavior is unchanged.
• With experimental.disable_vcs_diff: true, the desktop-layout web UI no longer emits GET /vcs/diff from the automatic review/diff path.
• During active web UI use with the flag enabled, no new /vcs/diff requests, heap warnings, or OOM crashes were observed in the logs.
• Ran bun --cwd packages/opencode typecheck.
• Ran bun --cwd packages/app typecheck.
• Ran the repository pre-push hook, which completed bun turbo typecheck successfully.

Screenshots / recordings

N/A. This is a config-gated behavior change, not a visual UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[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.