feat(pi): open the Braid panel on a single node

[powerfooI]

What changes

/braid accepts an optional node or execution after the job ID, opening the live panel focused on that one node:

/braid job-1 review          → latest invocation of node "review"
/braid job-1 <executionId>   → one exact invocation (e.g. an earlier loop iteration)

The focused view refreshes live and shows status, model, loop iteration, elapsed time, token usage, context size, tool calls, workspace, and the full output or error. Backspace returns to the whole graph; switching jobs also returns to it. Tab completes job handles, then that job's node IDs.

Why

Until now, the only way to see one node's details in Pi was to ask the model to call braid_status. The panel showed only the whole graph. This gives people a direct, live view of a single node.

Details

• The command treats everything after the job ID as the target, because core accepts any non-empty node ID, including IDs with spaces. It tries node IDs before execution IDs. IDs with surrounding whitespace, terminal controls, or a leading double quote use JSON string syntax; Tab inserts this safely and completion labels display controls as escapes. An unknown target fails immediately with a hint to open /braid <jobId>.
• If a live graph update removes the focused node, the panel says so instead of throwing.
• renderNodeResult is shared with focused braid_status results, which now show the same detail lines. A running node's elapsed time is computed at the read's observation time, so a saved tool result doesn't keep counting.
• Live tool-call figures are shown only when they belong to the execution being displayed. After a worktree is cleaned up, its checkpoint ref is shown instead of the deleted path, matching the graph view.

Verification

• npm run check, npm test (227 pass)
• npm run check:pi, npm run test:pi (96 pass), including new tests for:
  • node and execution focus, a running node's progress, and a removed node
  • Backspace and job switching
  • command parsing and error messages
  • Tab completion, including node IDs with spaces
  • renderer elapsed time, iteration, and workspace refs

This is independent of #52 and applies cleanly on either side of it.

🤖 Generated with Claude Code

Maintainer review fixes

• Escape terminal controls in completion labels and inserted values.
• Preserve exact node IDs with surrounding whitespace or quotes using JSON string targets, and report malformed quoted targets clearly.
• Add regression coverage using the real Pi SelectList and opening the completed target.
• Validation after fixes: npm run verify passed (227 core, 98 Pi, 35 DSH tests), including type checks, offline examples, and package-consumer smoke checks. Focused panel/render checks passed (23 tests); git diff --check passed.