fix(webui): make the keyless file-backed monitor report live progress

[AlcAI-Haven]

Summary

Fixes the keyless, file-backed WebUI monitor (open_dashboard MCP tool / arbor web) that a host coding agent drives via the arbor mcp tools. That monitor renders a session from disk with no live RunState or EventBus, but build_session_snapshot only populated the tree, counters and scores — so the page presented a frozen run.

Symptoms addressed:

• Runtime stuck at 0s and a blank improvement chart — no clock or score history was ever derived.
• Token / cache counters pinned at zero — those are owned by the host LLM harness, not Arbor, and can't be observed keylessly.
• Dead pipeline and empty Recent Activity — phase was hardcoded and there were no activity events.
• Black (invisible) node-modal title, and the title showed the entire multi-line hypothesis instead of a headline.
• Nodes "disappeared" when the dashboard was opened from the main checkout while the agent ran inside a linked git worktree (each worktree has its own .arbor/).

Changes

• Timing sidecar — the Node model intentionally stores no timestamps (and must stay byte-compatible), so a new mcp/session_timing.py maintains .coordinator/activity.json: a run clock, per-node started/finished, and a capped event log. session_ops records an event on add / update / prune / worktree_create / merge.
• session_source now derives elapsed_seconds, per-node runtime_seconds / finished_elapsed, the pipeline phase, running-node "agents", and best_score_history (which fixes the empty/flat improvement chart) from the sidecar.
• Tokens & Cache cards are hidden in keyless mode (snapshot flagged keyless) instead of showing misleading zeros, and the stat grid reflows.
• Worktree-stable sessions — .arbor/sessions is anchored at the main working tree when driven from a linked worktree (session_ops._stable_arbor_base, mirrored in export.resolve_session_dir); normal checkouts and non-git directories are unchanged.
• Modal fixes — the black title was caused by #overlay living outside the themed shell wrapper, so the bare <h2> inherited the browser-default color; color is now set on the overlay. Node titles render a one-line headline (oneLine), with the full hypothesis preserved in the modal's Details.

Testing

• Added tests for the sidecar-backed snapshot, activity recording, and the stable-base behaviour.
• Full suite: 345 passed, 2 skipped. The remaining failures (test_export path separators, test_worktree / test_zoo git-worktree & subprocess) reproduce on an untouched checkout and are pre-existing Windows-environment issues, not regressions.
• Verified end-to-end with Playwright against a generated keyless session: runtime ticks, Tokens/Cache cards absent, modal title legible and one-lined, pipeline shows a running step, Recent Activity populated, and the improvement chart draws a real curve.

🤖 Generated with Claude Code