fix(tui): rebuild dag inspector on the chat design language

[LeXwDeX]

Summary

Rebuilds the /dag inspector on the chat transcript's design language instead of the diff-viewer's frame vocabulary. Box-drawing lines (─┬├┴) are gone; the screen is now rail-and-panel blocks with two-column side padding, exactly as routes/session renders messages.

A grilling pass over the first attempt found three real defects, all fixed here.

1. Sidebar was not responsive (severe)

Chat gates its 42-wide sidebar on dimensions().width > 120 (routes/session/index.tsx). The first attempt rendered it unconditionally. Measured at 60 columns:

|  wave 1· 1 node        |   ← gap collapsed
|    ✓ coll build        |   ← node name truncated to "coll"
|    ⠋aGeneralPur        |   ← name reduced to "a", spinner glued on

Now gated on the same threshold. Below it the sidebar is dropped and the summary block carries workflow 1/2 so the left/right cursor stays legible. Locked in by a new regression test asserting the sidebar appears at 130 columns and is absent at 100.

2. Selection used the "unfocused" highlight

dialog-select uses primary + selectedForeground() for the active row and degrades to backgroundElement only when focus moves away. The first attempt used backgroundElement for the focused state — semantically "selected but unfocused", and only an 8% luminance step (#0a0a0a → #1e1e1e), so the cursor was nearly invisible on low-contrast terminals.

Node cursor (what return acts on) now uses primary + selectedForeground(); the secondary workflow selection keeps backgroundElement. One strong highlight on screen, no ambiguity about what return targets.

3. Fixed layout for the empty state

Panels keep a fixed layout: the sidebar is present whenever the terminal is wide and simply empty when there is nothing to list. The empty state also gained a Run /dag-flow <task> … hint — previously it was a dead end, which is what made an earlier UI change look like "nothing changed" (the empty state is byte-identical across versions since it was never restyled).

Cursor-only interaction model

Command	Before	After
dag.down / dag.up	j,down / k,up	down / up
dag.next_workflow / dag.previous_workflow	l,right,tab / h,left	right / left
dag.close	escape,q	escape
dag.enter	return	return
dag.pause / resume / step / cancel	p / r / s / x	none

The four control operations gain namespace: "palette" so they stay reachable through the command palette — without it, unbinding them would have made the capability unreachable, since route commands registered via useBindings are not palette-visible by default. A test pins this. The footer is now a single uncrowded row: enter open session escape close.

Test plan

• tsgo --noEmit clean in packages/tui
• Full TUI suite: 233 pass / 0 fail (43 in the DAG files, up from 41)
• Rewrote the three pressKey("p") tests to invoke dag.pause directly, and replaced the contextual-footer assertion with one that verifies control hints never appear
• Lint ratchet tightened 4704 → 4690: a runCommand test helper collapses ten duplicated command-context assertions into one, a net reduction of 14 warnings