Add workspace grep match anchors

[chubes4]

Summary

• Add stable-enough match_id, preview text, and workspace_read-ready read_args to local and remote workspace_grep matches.
• Add actionable workspace_edit old_string not found suggestions with nearby snippets for local and remote workspace backends.
• Extend focused smoke coverage for grep anchors and edit miss context.

Closes #310.

Tests

• php tests/smoke-workspace-grep.php
• php tests/smoke-workspace-edit-context.php
• php -l inc/Workspace/WorkspaceReader.php && php -l inc/Workspace/WorkspaceWriter.php && php -l inc/Workspace/RemoteWorkspaceBackend.php && php -l tests/smoke-workspace-edit-context.php
• git diff --check
• homeboy lint --path . (fails on existing repository-wide PHPCS findings unrelated to this change)

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the workspace grep anchor metadata, edit failure context, smoke coverage, and local verification; Chris remains responsible for review and merge.