Fix Auto-Sync Workspace Registration Issue

Issue: #2288

Problem: The zsh plugin automatically syncs workspaces in the background on every directory change, causing unintended parent directories to be registered as workspaces. When users run forge workspace list, ancestor directories appear as "Current" instead of the actual working directory.

Root Cause: _forge_start_background_sync() in shell-plugin/lib/helpers.zsh:114-131 unconditionally runs forge workspace sync without checking if the workspace or its ancestors are already registered.

Objective

Prevent auto-sync from creating unintended workspace registrations while maintaining the convenience of automatic syncing for already-indexed workspaces.

Implementation Plan

Phase 1: Add Workspace Detection Command

Add new CLI subcommand forge workspace is-indexed [PATH] that checks if a path (or any ancestor) is already registered as a workspace
Implement in crates/forge_main/src/cli.rs - add IsIndexed variant to WorkspaceCommand enum
Add handler in crates/forge_main/src/ui.rs that calls existing is_indexed() service method
The command should return exit code 0 if indexed, exit code 1 if not indexed
Output should be silent by default (only exit code matters for scripting)
Add optional --verbose flag to show which workspace was found (exact or ancestor)

Phase 2: Update Zsh Background Sync Logic

Modify _forge_start_background_sync() in shell-plugin/lib/helpers.zsh:114-131
Before running sync, call $_FORGE_BIN workspace is-indexed "$workspace_path"
Check exit code: if non-zero (not indexed), skip the sync and return early
Only proceed with background sync if exit code is 0 (workspace or ancestor is already indexed)
Add debug logging (when FORGE_DEBUG=true) to indicate when sync is skipped

Phase 3: Handle Initial Workspace Registration

Ensure manual forge workspace sync still works for first-time registration
Update documentation to clarify the auto-sync behavior (only syncs already-registered workspaces)
Consider adding a helper message when users first cd into a new directory that suggests running forge workspace sync to enable auto-sync

Phase 4: Update Workspace List Display

Enhance on_list_workspaces() in crates/forge_main/src/ui.rs:3266-3311 to distinguish between exact match and ancestor match
When current workspace is an ancestor match, display as [Current via ancestor] or similar
Show the actual current directory path in the output when it differs from the workspace path

Verification Criteria

Before Fix

Run cd /Users/username (parent directory)
Run forge workspace sync once
Run cd /Users/username/Documents/Projects/forge (subdirectory)
Trigger zsh accept-line (press Enter on any command)
Run forge workspace list
Verify: /Users/username/Documents/Projects/forge is incorrectly registered as a workspace

After Fix

Clean database and repeat above steps
Run cd /Users/username/Documents/Projects/forge
Trigger zsh accept-line (press Enter)
Run forge workspace list
Verify: Only /Users/username appears (no auto-sync of subdirectory)
Verify: List shows [Current via ancestor] or similar indication
Run explicit forge workspace sync in subdirectory
Verify: Now subdirectory is registered and will auto-sync on future visits

Edge Cases

Test behavior when no workspace exists at all (should not auto-sync)
Test behavior in deeply nested directories (should find closest ancestor)
Test behavior when FORGE_SYNC_ENABLED=false (should still respect flag)
Test manual sync in new directory (should still work)

Potential Risks and Mitigations

Risk 1: Breaking Existing User Workflows

Mitigation: Users who rely on automatic workspace creation will need to run forge workspace sync once per workspace. This is a one-time migration cost for better UX long-term.

Risk 2: Performance Impact

Mitigation: The workspace is-indexed check is a fast database query (already exists in is_indexed() method). It should add negligible overhead.

Risk 3: Race Conditions

Mitigation: The check and sync are not atomic, but this is acceptable - worst case, a sync happens when it shouldn't have. No data corruption risk.

Alternative Approaches

Alternative 1: Opt-in Auto-Sync

Instead of checking if workspace exists, require users to explicitly enable auto-sync per workspace via a flag or configuration file.

Pros: More explicit control
Cons: More complex UX, requires additional configuration management

Alternative 2: Display-Only Fix

Just improve the workspace list display to show [Current via ancestor] without changing auto-sync behavior.

Pros: Simpler implementation, no behavior changes
Cons: Doesn't solve the root problem of unintended workspace registrations

Alternative 3: Auto-Sync with Depth Limit

Only auto-sync directories within N levels of an existing workspace.

Pros: Prevents deep directory pollution
Cons: Arbitrary limit, still creates unintended workspaces

Dependencies

No external dependencies
Existing is_indexed() method in WorkspaceService can be reused
Zsh plugin already has access to $_FORGE_BIN for CLI calls

Testing Strategy

Unit Tests: Add tests for workspace is-indexed command in CLI tests
Integration Tests: Test zsh function behavior with mocked CLI commands
Manual Testing: Follow verification criteria above
Regression Testing: Ensure existing workspace sync functionality unchanged

Documentation Updates

Update shell-plugin/README.md to explain auto-sync behavior
Add note about one-time workspace sync requirement for new workspaces
Document FORGE_SYNC_ENABLED environment variable
Update workspace command documentation with new is-indexed subcommand