feat(tui): FR-TUI-P polish + bugfixes (muted palette, hero, metric strip, SIGWINCH, attach hardening)

[linhdmn]

Closes #146.

Implements all 12 FR-TUI-P items as a pure-Go change confined to internal/tui/ (+ docs/TUI.md). No new runtime deps.

What changed (FR-TUI-P-01..12)

• P-01 attach-resume hardening — attachLocked now guarantees resume on every child-death path: nonzero exit (attach exited (N)), signal death (attach child died (signal N)), and panics inside the env (attach crashed: …). The alternate screen is re-entered, raw mode re-armed, the frame full-repainted and a fresh poll fired; the dashboard never process-exits because the attach child died. A synchronous safeDrawLocked before the async poll pins the note on screen before it can be cleared.
• P-02 poisoned-shape tests — new poisonDaemon httptest server serves arrays-where-objects and objects-where-arrays on /status, /agents, /history, /sessions; FetchSnapshot must normalize/degrade to empty payloads and render without panic.
• P-03 failed_recent never FAILED — regression tests pin that failed_recent>0 with all panes idle + closed circuit aggregates to IDLE and never paints FAILED (code was already correct; the strip now renders the count as a historical amber Nf recent suffix beside live active counts).
• P-04 SIGWINCH — LoopEnv.Sigwinch() added (nil-able, like Sigint); TermEnv subscribes signal.Notify(SIGWINCH); the loop repaints promptly on resize instead of waiting for the next 2s poll. Run-level test mutates cols and fires the channel, asserting the sanctioned reflow full-clear.
• P-05 hero line — new heroLines: the selected (or first) running pane's id/phase/elapsed/worker with an indeterminate pulse bar driven by the spinner frame — no fabricated percent (the snapshot carries no fraction field; issue open-question 1 resolved to indeterminate). Idle shows a next-action cue. Folded with the loop-phase iteration row.
• P-06 dense metric strip — one row: queue meter + Np/Nc/Nd, live runs, activity sparkline; uptime/herdr/visibility demoted to the dim tail.
• P-07 worker cards — kept (chip/title/elapsed/attach-hint/▸ already present; trim pinned by existing selection test).
• P-08 narrow stacking — below 80 columns worker/queued cards stack full-width instead of crushing 2-up; ≥80 keeps 2-up. Overflow pinned at 78 cols.
• P-09 muted palette — ansi.go rewritten: rebindable SGR vars resolving to pilot's chart colors (steel #7eb8da, sage #7ec699, rose #d48a8a, amber #e0af68, slate #3d4450, gray) under COLORTERM=truecolor|24bit, with a 16-color fallback otherwise. Magenta retired. StatusColor/ChipFor carry the palette to every card.
• P-10 redraw guarantees — tests pin: identical consecutive frames emit zero row writes (no 2J), a one-row change rewrites exactly that row; the resize full-clear is the loop's one sanctioned clear.
• P-11 wide-glyph clamps — covered by existing ClampLine/VisibleLen CJK/fullwidth/astral tests (still green).
• P-12 footer/help — footer hint folds by exact width tiers (full ≥116, mid ≥96, compact ≥62, else help/quit) so [q] quit is never clamped off — the full hint previously overflowed the default 100-col terminal. The ? help overlay keeps its bottom rows, so the quit row is visible even at 24×80 (pinned at 24×80 and 10 rows).

Evidence

$ go test ./internal/tui/ -count=1
ok  	github.com/FreePeak/devagent/internal/tui	2.403s

$ gofmt -l internal/tui/   # empty
$ go vet ./internal/tui/   # clean

20 new contract tests in polish_contract_test.go (P-01/02/03/04/05/08/10/12) + existing suite green. docs/TUI.md gains the Layout & visual language (FR-TUI-P) section (layout diagram, palette table, attach-resume + redraw contracts) and the Architecture table note on the sanctioned resize clear.