Bring cumulative RemCodex improvements upstream

[john-craig]

Summary

This pull request brings the cumulative RemCodex improvements from the john-craig/remcodex fork into lupishan/remcodex. The source of truth remains the Gitea repository; the fork is configured as a public push mirror.

Changes

Session and project workflows

• Added persisted parent-session relationships and a nested/expandable session tree in the web UI.
• Enforced directory-hierarchy parenting: parents must live at the same or an ancestor working-directory path.
• Added working-directory project lookup, project creation, session reuse, session listing, and session resumption.
• Added session descriptions, deduplicated tags, arbitrary JSON metadata, and external/API/MCP update support.
• Added starting prompts for profiles, including one-shot prompt composition and a temporary UI banner.
• Added stable openable session links to MCP responses.

Interactive Codex app-server support

• Added discovery and synchronization of interactive Codex app-servers through the registry under the runtime directory.
• Added remote app-server execution, rollout/thread synchronization, session attachment, and unattended approval behavior.
• Added the Python remcodex-tui wrapper. Bare invocation starts a discoverable Unix-socket app-server and launches the TUI through codex --remote; explicit Codex arguments pass through unchanged.
• Added green/red liveness indicators for sessions currently connected to, or previously disconnected from, an interactive TUI.

MCP and diagnostics

• Added authenticated RemCodex MCP server support and smoke-test tooling.
• Added MCP tools for profiles, projects, session creation/reuse, directory listing, resumption, metadata updates, history, approvals, and session links.
• Preserved child-process stderr and richer failure diagnostics, including command, working directory, execution mode, exit code, and truncation state.
• Added client-side WebSocket/session synchronization diagnostics and a mock app-server/Playwright reproducer for dropped UI replies.

Web UI and deployment

• Added the RemCodex workspace UI, session navigation, timeline rendering, approvals, voice-note capture, profile selection, and responsive/mobile presentation.
• Added a RemCodex-styled /directory page driven by validated REMCODEX_DIRECTORY_INSTANCES JSON configuration, with links to multiple RemCodex instances.
• Added optional REMCODEX_HOSTNAME support so API/MCP session links can use the hostname seen by end users, while retaining relative links when unset.
• Added startup configuration examples, packaging support, assets, and the required Nix flake/check scaffolding.

Validation

• npm run build
• JavaScript/TypeScript tests: 25 passing
• Python unit tests: 3 passing
• node --check web/app.js

The Gitea source and public GitHub fork are configured to stay synchronized through the Gitea push mirror.