feat(app-server): add filesystem symlink semantics

[nornagon-openai]

Why

Codex Apps adapts app-server filesystem operations to its execution-host interface. Callers need to distinguish a symlink from its target both for explicit metadata queries and directory listings.

What

• Add nullable followSymlinks to fs/getMetadata, defaulting to target-following behavior.
• Add lstat-style metadata support across direct, sandboxed, and remote exec-server transports.
• Add isSymlink to fs/readDirectory entries while preserving target-derived isDirectory and isFile values.
• Update generated protocol schemas, API documentation, and local/remote behavior coverage.

This PR is intentionally limited to symlink semantics. File-size metadata and exclusive copies are independent changes.

Validation

• just write-app-server-schema
• just test -p codex-app-server-protocol
• just test -p codex-exec-server -E 'test(file_system_get_metadata_reports_symlink_targets) | test(file_system_read_directory_reports_symlinks)'
• just test -p codex-app-server -E 'test(fs_get_metadata_reports_symlink) | test(fs_read_directory_reports_symlink_entries)'
• just fix -p codex-file-system -p codex-exec-server -p codex-app-server-protocol -p codex-app-server
• just fmt