(session-state): pure domain module, single DOM writer, enforced by lint

[devsuitup]

Steps 1-2 of #246 (migration order in the issue). Steps 3-5 follow in separate PRs.

What

• public/session-activity.js is split: it keeps the Maps/Sets, setActivity, purgeActivityFor, rekey/reconcile; the new public/session-activity-dom.js holds sessionItemEl, applyActivityClasses and four setters that are the only code allowed to write .cli-busy / .needs-attention / .response-ready / .has-busy-agents. The direct writers in app.js (3) and sidebar.js (4) are routed through them.
• public/session-state.js: pure, dual-loadable domain module. createSessionState(kind).apply(event).snapshot() with liveness and attached as two separate facts (lifecycle decision recorded on the issue), exclusivity of busy / waitingForInput / attention, and the priority order. renderSessionIcon(snapshot) gives classes, glyph and title for one icon slot; only its classes are consumed for now, behind applyActivityClasses, local-pty only. Local DOM output is unchanged.
• Enforcement: no-restricted-syntax in eslint.config.js (classList add/remove/toggle with those class names outside the projection file) and test/session-state-boundary.test.js (no document / window / electron / ipcRenderer in the domain source).
• .ai/contexts/session-state.md (shape, ports, enforcement, migration status), linked from the contexts README and the orientation table.

Two deviations from the issue text, documented in the context note: a responseReady field (the "finished, not yet seen" rung is distinct from waitingForInput), and the seen/unseen flag stays caller-supplied (armReady) because attached means "has a PTY", not "is the focused tab".

Test infra changes: loaders updated for the split files; one source-regex pin (sidebar-busy-agents-tint.test.js) rewritten against the real module; one pin in running-indicators.test.js updated to the new call shape.

Proof

• Lint rule: an injected classList.add('cli-busy') in app.js fails npx eslint public/app.js with one no-restricted-syntax error; removed, 0 errors.
• Mutation: reordering PRIORITY turns three priority tests red.
• task check: 1176 + 119 pass, 0 fail, 8 pre-existing skips.

[devsuitup]

Review pass (adversarial, six axes): local DOM output, remote rows, boundary test, load order, dual-load clean. One finding kept: the lint rule only matched classList.add/remove/toggle('<literal>'), so className =, innerHTML, setAttribute('class', …) and classList['toggle'](…) went through. Second commit broadens it (string and template literals in those sinks, computed classList[...] refused outright); probe: 6/6 bypass shapes red, unrelated class green, 0 errors on the renderer. A class name held in a variable stays out of lint's reach — stated in the context note. Also noted there: the busy/response-ready tie now resolves as busy (was response-ready), unreachable given setActivity's exclusivity.