Task: Add LLM snapshot summarisation skill and sprint timeline storage

Add a compass timeline snapshot command and an agile-add-timeline-snapshot skill so the LLM can produce structured 20-minute activity buckets and store them in the sprint's timeline/ folder. Each snapshot is a scan-first org file with Stories, Tasks, Captures, PRs, Problems/suspicious-decisions, and an Audit section. A companion recipe documents the workflow.

• Snapshots are scan-first: bullet points and tables, not prose. We have too much information already — the snapshot must be readable in seconds.
• Buckets: stories raised, tasks raised, stories/tasks worked on (per environment), problems, suspicious LLM decisions.
• Every row links to the document by id so detail is one click away.
• An audit section per snapshot: window-scoped bookkeeping drift (states not updated, missing environment stamps, merged PRs with open tasks), reusing the sprint audit checks.
• Stored under the sprint's timeline/ folder as <ISO-from>-<ISO-to>.org.
• Bucket size is tunable and treated as an experiment: start around 20 minutes, adjust against real activity. A bucket with too many events to scan quickly is a failure — prefer splitting busy windows (event-count cap) over letting buckets grow.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

• Added _cmd_snapshot, _write_snapshot, _find_sprint_timeline_dir, _snapshot_filename, _doc_link, _event_desc to compass_timeline.py. The snapshot subcommand resolves a window, fetches origin/main, buckets doc and PR events, auto-detects the current sprint's timeline/ dir, generates a UUID per snapshot, and writes the structured org file.
• Event-count cap of 20 per bucket enforced with a warning on overflow.
• Audit section auto-flags items closed without #+environment: stamp.
• Created doc/llm/skills/agile-add-timeline-snapshot/SKILL.org and deployed via compass build skills.
• Created doc/recipes/compass/how_do_i_create_a_timeline_snapshot.org.
• Added skill to doc/llm/skills/claude_code_skills.org.