fix(opencode): pass --dir to opencode so agents run in the worktree, not main repo

[jregeimbal]

Summary

When opencode is launched inside a git worktree, it follows the .git file pointer back to the main repository root, causing all agent file operations to target the main repo instead of the isolated worktree. Passing --dir <worktree> prevents this gitdir resolution.

Fixes #530.

Motivation

Worktree isolation exists to keep each cluster's changes sandboxed. opencode's internal git detection walks .git (which in a worktree is a file like gitdir: /main/.git/worktrees/...) up to the main repo root, silently defeating the isolation — the agent reads and modifies files in the main repo rather than the worktree.

The fix is to prefer --dir over --cwd when opencode supports it. Unlike --cwd, --dir tells opencode explicitly which directory to treat as root and bypasses the gitdir pointer walk.

Changes

• src/agent-cli-provider/types.ts: Add supportsDir to OpencodeCliFeatures and CliFeatureOverrides
• src/agent-cli-provider/adapters/opencode.ts: Detect --dir in detectCliFeatures() via help text scan; in addOpencodeOptionalArgs(), pass --dir <cwd> when supportsDir is true, falling back to --cwd for older opencode versions that lack the flag
• tests/provider-cli-builder.test.js: 5 new regression tests covering:
  • --dir is passed when supportsDir is true (the fix)
  • --cwd is used as fallback when supportsDir is false
  • commandSpec.cwd is always set regardless of CLI flag support
  • detectCliFeatures correctly detects supportsDir from help text
  • detectCliFeatures correctly rejects supportsDir when --dir is absent

Testing

• Unit tests added (tests/provider-cli-builder.test.js — 5 new tests, all passing)
• Full test suite passes (1244 tests)
• Linting passes (0 errors)
• Type checking passes

Breaking Changes

None. --dir is only passed when detectCliFeatures() confirms opencode supports it; older versions continue to receive --cwd as before.

Checklist

• Tests pass (npm test)
• Linting passes (npm run lint)
• Type checking passes (npm run typecheck)
• Documentation updated (JSDoc on affected functions)
• CLAUDE.md not affected (no architecture change)