Add workspace grep tool

[chubes4]

Summary

• Adds a read-only workspace_grep tool / datamachine/workspace-grep ability for searching workspace files by regex with optional path, include glob, result cap, and context lines.
• Implements local filesystem search for primary/worktree handles and GitHub API-backed search for remote registered primary/worktree handles, including pending remote worktree edits.
• Fixes remote primary reads to avoid passing an empty ref so preloaded primary handles can read/search default-branch content before worktree creation.

Fixes #306.

Tests

• php -l inc/Abilities/WorkspaceAbilities.php && php -l inc/Cli/Commands/WorkspaceCommand.php && php -l inc/Tools/WorkspaceTools.php && php -l inc/Workspace/RemoteWorkspaceBackend.php && php -l inc/Workspace/WorkspaceReader.php && php -l tests/smoke-remote-workspace-backend.php && php -l tests/smoke-workspace-grep.php
• php tests/smoke-workspace-grep.php && php tests/smoke-remote-workspace-backend.php
• git diff --check origin/main...HEAD
• homeboy lint data-machine-code --path /Users/chubes/Developer/data-machine-code@fix-workspace-grep --changed-since origin/main failed with existing PHPCS findings in changed files (96 findings after grep-specific cleanup), including pre-existing alignment warnings around WorkspaceAbilities.php cleanup schemas and WorkspaceCommand.php cleanup rendering.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the workspace grep ability/tool/backend changes and smoke tests; Chris remains responsible for review and validation.