feat: cli timeline <session_id> generates markdown session log

[jakduch]

What does this add and why do you believe it belongs in this dashboard?

Adds python3 cli.py timeline <session_id> [--out FILE], a one-shot generator that replays a single Claude Code session as a time-ordered Markdown log: header (project, branch, full UUID, first/last timestamps, turn count, totals, cost), a chronological list of turns (HH:MM:SS · model · tool · in/out tokens → cost), and a tool histogram. Accepts any unique session-id prefix (4+ chars), errors clearly on unknown or ambiguous ids, and writes to stdout by default or to a file with --out.

The dashboard is great for grazing across all sessions, but when something goes wrong in one session you want a flat, greppable, shareable artifact — not a screenshot of a webview. This gives you exactly that with zero new dependencies: a postmortem document you can paste into a GitHub issue, attach to a bug report, diff between two runs, or feed back into Claude when asking "what did I do here?". It also makes the existing turn-level data the scanner already collects directly addressable from the CLI, which is the natural home for ad-hoc, scriptable workflows.

Checklist

Code correctness

• All calcCost() calls pass 6 arguments: (model, inp, out, cache_read, cache_creation, cache_1h)
• JavaScript template literals use bare backticks (`), not escaped ones (\`)
• No JS variables referenced before they are defined
• No new third-party dependencies introduced

Tests

• python3 -m unittest discover -s tests -v — all passing
• python3 -m unittest tests.test_browser -v — all passing
• New behaviour is covered by at least one test

Scope

• This is a single concern — one feature or fix per PR
• Only touches existing files (dashboard.py, scanner.py, cli.py, pricing.py, cowork.py, tests/) — or I've explained below why a new file is needed

tests/test_cli_timeline.py is a new file under tests/ covering the new cmd_timeline behaviour (unknown/ambiguous ids, header rendering, turn ordering, tool histogram, --out file writing). No production code lives outside the existing cli.py.