feat: multi-provider session scanning for discover and learn

[sahilmgandhi]

A followup to #254, wanted to add more provider support for rtk discover and rtk learn. We don't need it for rtk gain since all rtk commands automatically populate the sqllite db with usage data.

Summary

• Add session scanning support for Codex CLI, Cline, and Cursor alongside existing Claude Code provider
• New --tool / -t flag on rtk discover and rtk learn to filter by tool (claude, codex, cline, cursor)
• Report header now shows which tools were scanned; "Bash commands" → "shell commands"

New Providers

Provider	Session Format	Location
CodexProvider	JSONL rollout files	~/.codex/sessions/ ($CODEX_HOME override)
ClineProvider	JSON conversation history	~/.cline/tasks/ + VS Code extension storage
CursorProvider	SQLite databases	state.vscdb (desktop) + store.db (agent CLI)

Shared Infrastructure

• ToolSource enum with short_name() / display_name()
• build_providers(tool_filter) factory with validation
• cutoff_from_days(), is_recent(), join_tool_uses_with_results() shared helpers
• OUTPUT_PREVIEW_CHARS constant (was magic 1000 in 4 places)

Design Decisions

• Providers that aren't installed return Ok(vec![]) — no errors for missing tools
• Cursor parsing is best-effort (undocumented format, read-only SQLite access)
• Project filtering: Claude encodes CWD internally; Codex peeks at SessionMeta; Cline/Cursor don't have project-scoped sessions
• No new dependencies — rusqlite already existed for tracking

Test plan

• 36 new unit tests across all 3 providers + trait tests (446 total, all passing)
• cargo fmt --all --check clean
• cargo clippy --all-targets — 0 errors
• Manual: rtk discover --all scans all installed tools
• Manual: rtk discover --tool claude filters to Claude only
• Manual: rtk discover --tool invalid returns helpful error
• Manual: rtk discover --format json includes tools_scanned field

  ┌────────────────────┬───────────────────────────────────┬────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
  │        Test        │              Command              │                                                                 Result                                                                 │
  ├────────────────────┼───────────────────────────────────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
  │ Scan all tools     │ rtk discover --all                │ PASS — Header shows Tools: Claude Code, Codex CLI, Cline, Cursor. Found 319 sessions (316 Claude + 3 Cline). Reports "shell commands". │
  ├────────────────────┼───────────────────────────────────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
  │ Filter by tool     │ rtk discover --tool claude --all  │ PASS — Header shows Tools: Claude Code. Found 316 sessions (Claude only).                                                              │
  ├────────────────────┼───────────────────────────────────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
  │ Invalid tool error │ rtk discover --tool invalid --all │ PASS — Error: Unknown tool 'invalid'. Valid options: claude, codex, cline, cursor (exit code 1).                                       │
  ├────────────────────┼───────────────────────────────────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤
  │ JSON tools_scanned │ rtk discover --all --format json  │ PASS — JSON includes "tools_scanned": ["Claude Code", "Codex CLI", "Cline", "Cursor"].                                                 │
  └────────────────────┴───────────────────────────────────┴────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘