Add Codex provider support

[lakshya-dhariwal]

Summary

Adds Codex CLI as a first-class ccview provider alongside Claude Code and OpenCode.

The implementation reads Codex's portable local data layout rather than machine-specific paths:

• ~/.codex/sessions/YYYY/MM/DD/*.jsonl for rollout message history
• latest ~/.codex/state_*.sqlite for thread metadata, project grouping, token totals, git branch, model info, and subagent edges
• ~/.codex/session_index.jsonl as a title/timestamp fallback
• ~/.codex/shell_snapshots/*.sh as optional linked shell snapshot files

Features

• Adds a CodexProvider that implements the existing provider interface.
• Groups Codex sessions by cwd/project and sorts by recent activity.
• Renders Codex user/assistant messages, reasoning summaries, function calls, custom tools, web search calls, image generation calls, tool outputs, and token usage.
• Maps Codex thread-spawn edges to ccview subagents, matching the existing Claude/OpenCode subagent UX.
• Shows Codex token totals and shell snapshot badges in the TUI and web UI.
• Makes provider discovery and web tree merging generic across Claude Code, OpenCode, and Codex.
• Allows direct --file/--export usage for Codex rollout JSONL files.
• Updates docs with the Codex data footprint using version-tolerant state_*.sqlite / goals_*.sqlite patterns.

Notes

The Codex provider treats SQLite/thread metadata and shell snapshots as optional. If a Codex installation only has rollout JSONL files, ccview still loads conversation history and degrades gracefully.

Testing

• go build -o ccview .
• go test ./...
• Local smoke test with ./ccview --web --port 3334
  • verified /api/tree includes Codex projects/conversations/subagents/token totals/shell snapshots
  • verified /api/messages?source=codex renders Codex messages, token usage, and tool cards
  • verified ./ccview --export /tmp/ccview-codex-export.html --file <codex-rollout.jsonl>