fix(tui): restore the dag inspector's own structure, keep only the vocabulary

[LeXwDeX]

Summary

The previous revision (#137) transplanted chat's structure instead of learning its design language. This PR keeps the vocabulary and gives the inspector its own structure back.

What was wrongly imported

Removed	Its meaning in chat	What it was doing here
42-wide right sidebar + backgroundPanel	container for ambient session metadata (MCP, todos, context) — optional	held the workflow list, which is primary navigation
width > 120 gate	sized for that optional metadata	made primary navigation vanish on narrow terminals
SplitBorder rail-and-panel block	marks one utterance in a stream	wrapped the workflow summary, which is persistent state

The tell was the symptom: the workflow list disappearing below 120 columns. #137 treated that as "responsive not implemented" and copied chat's breakpoint to fix it, entrenching the transplant rather than questioning it. Primary navigation should not have a "disappeared" state at all.

Structure now derives from what this screen is

A full-screen inspector: title row → always-present navigation pane → content pane → detail pane → footer. Navigation width is clamp(18, 32, width * 0.3), derived from the pane's own needs, so it narrows instead of disappearing:

width=110                                        width=70
|  DAG Evidence pipeline        2 workflows  |   |  DAG Evidence pipeline    2 workflows  |
|                                            |   |                                        |
|  • Evidence pipeline  1/4   running · 3 no |   |  • Evidence pi 1/4   running · 3 nodes |
|  • Second workflow    0/2                  |   |  • Second work 0/2                     |
|                       wave 1 · 1 node      |   |                wave 1 · 1 node         |
|                         ✓ collect build    |   |                  ✓ collect build       |
|                                            |   |                                        |
|                       wave 2 · 2 nodes     |   |                wave 2 · 2 nodes        |
|                         ⠋ analyze General. |   |                  ⠋ analyze GeneralPurp |

Only the vocabulary is borrowed

• Regions marked by the panel surface + padding rhythm rather than drawn box characters — which also settles the original "the bottom rule looks misaligned" complaint by removing the rules entirely
• Token semantics: text body, textMuted metadata, backgroundElement secondary selection, primary + selectedForeground() for the cursor, status colours for state
• Bold reserved for identity (DAG) and section headers (wave N); body and metadata never bold

Kept from #137: cursor-key convergence (↑↓ nodes, ←→ workflows, return, escape), the selection colour layering, the empty-state guidance, and the tightened lint ratchet.

Test plan

• tsgo --noEmit clean; prettier --check clean
• oxlint on the changed component: 0 warnings
• Lint ratchet at 4690: PASS
• Full TUI suite: 233 pass / 0 fail
• The width regression test now asserts the inverse invariant — the workflow list is present at 130, 100 and 70 columns — with a comment recording why the previous gate was wrong, so the mistake cannot silently return