v0.5.0: remove duration_ms and files tracking

[fjgbue]

What's in v0.5.0

Bug fixes / cleanup

• Remove duration_ms — was always null for hook-recorded sessions (the majority). Removed from run.json, both recorders, sessions table column, and detail panel.
• Remove files tracking — file.read events and files.reads[] array were stored in run.json but never surfaced in the dashboard and were unreliable.

Analytics & visualizations (all use Recharts, already a dependency)

Per-turn token breakdown

• New Turns tab in session detail showing a stacked bar chart: input (blue) / cache write (amber) / output (green) per turn
• transcript.mjs now extracts per-turn usage into a turns[] array stored in run.json
• Cache reads shown as a note (not stacked — they repeat context and aren't billed as new tokens)
• Requires sessions recorded after this update; old sessions show a graceful message

Top sessions by cost

• New Top Sessions by Cost horizontal bar chart in the left column
• Shows up to 10 sessions ranked by cost, truncated description as label
• Clicking a bar selects the session in the detail panel

Cache hit rate

• New Cache Hit Rate line chart (daily, green) showing cache_reads / (cache_reads + fresh_input) over time
• Dashed reference line at 80% (good baseline)
• Shown side-by-side with the Cost per Day bar chart
• New 5th metric card in the strip showing the overall cache hit rate % for the selected period

Context bloat warning

• Session detail now warns when cache_creation_tokens > output_tokens × 8 — indicates the model loaded a large context but produced little output (expensive pattern)

Layout

MetricsStrip:  Total Cost | Total Tokens | Sessions | Avg Cost | Cache Hit Rate
Charts row:    [Cost per Day ████]  [Cache Hit Rate ———]
               [Top Sessions by Cost ████████████]
               [Sessions table]

Test plan

• tt dash — confirm 5 metric cards, two charts side-by-side, top cost chart
• Click a bar in the top cost chart — session should be selected in detail panel
• Record a new session — confirm run.json has turns[] array
• Open detail on a session with turns — confirm Turns tab appears with stacked chart
• Check context bloat warning on a session with high cache_creation / low output

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