fix(readers): repair amplifier/claude-code, add codex + copilot

[Joi]

Summary

Two existing readers had drifted out of sync with their source formats, so nightly runs were silently producing zero signals against real data. This PR restores both readers and adds two of the planned readers already listed in the README.

• amplifier — Discover both the legacy flat layout and the current nested layout (<projects>/<project>/sessions/<sess>/{transcript,events}.jsonl). Newer Amplifier sessions write events.jsonl instead of transcript.jsonl; the loader synthesizes Schema-B messages from prompt:submit (user), llm:response text blocks (assistant), and tool:post (tool, with structured success/output preserved so detect_errors still fires on success: false).
• claude-code — Unwrap the {type, message: {role, content}} wrapper that real Claude Code lines now use. Legacy flat {role, content} lines continue to parse. Session-meta lines (last-prompt, permission-mode, ai-title, attachment, file-history-snapshot) are skipped.
• codex (new) — Walks ~/.codex/sessions/YYYY/MM/DD/rollout-*.jsonl and emits user + assistant messages from response_item lines whose payload.type is message. Drops developer (system prompt) and non-message response items (reasoning, function_call, custom_tool_call).
• copilot (new) — Walks ~/.copilot/session-state/<uuid>/events.jsonl and emits messages from user.message and assistant.message events. Session metadata (session.*, assistant.turn_*, system.message) is skipped.

Wire-up

• jilog-review/src/readers/mod.rs: re-export CodexReader, CopilotReader.
• jilog/src/config.rs: add type = "codex" and type = "copilot" variants with sensible default paths (~/.codex/sessions, ~/.copilot/session-state).
• README.md: refresh the Readers table and the Quick Start config example.

Test plan

• cargo test -p jilog-review — 83/83 pass, including 6 new unit tests
• cargo build — clean
• Smoke test against a real multi-harness machine: 24h window jumped from 0 sessions / 0 signals to 67 sessions / 101 signals (69 corrections, 23 errors, 8 workarounds, 1 deferral, 1 P0 alert)
• CI green

New tests cover:

• amplifier: legacy flat layout, current nested layout, events-only sessions, both-present preference, events synthesis (user + assistant + tool with success: false)
• claude-code: wrapped message field + meta-line skipping
• codex: dated rollout discovery, role filtering (user/assistant kept, developer dropped, non-message response items dropped)
• copilot: session-state discovery, role filtering (user.message and assistant.message kept, all session metadata dropped)