feat: app://instructions + trim /settings panel to workspace-config only

[mgoldsborough]

Summary

Wires Collateral Studio up to NimbleBrain's bundle-side custom-instructions contract (`app://instructions`) and resolves the duplicate edit surfaces between the platform's `/settings/workspace/apps/synapse-collateral` panel and the in-app Settings drawer.

Principle applied

`/settings/workspace/apps/` is the canonical home for workspace-wide config that affects agent behavior even when nobody's in the app's UI. The in-app UI is for authoring (documents, templates, components, assets). The two surfaces should not duplicate each other.

Currently in /settings	Verdict
Brand Voice → Custom Instructions	Keep — now wired to `app://instructions`
Theme (colors + fonts)	Keep — workspace-wide brand identity
Reusable Components	Drop — promoted to in-app tab
Assets	Drop — already had a full in-app `AssetsView`

Server changes (`src/mcp_collateral/`)

• New `app://instructions` resource returning `voice.md` body. Platform wraps non-empty output in `` automatically.
• Drop the manual voice → skill-resource splice (`` placeholder removed from `SKILL.md`).
• `set_voice` enforces an 8 KiB UTF-8 cap (`MAX_VOICE_BYTES`), returns a structured tool error on overflow, and clears the file on empty input. `store.clear_voice()` added for the post-condition.
• Inline settings HTML trimmed to Theme + Custom Instructions. Reads via `readResource("app://instructions")`, writes via `set_voice`. UTF-8-byte-aware character counter; Save disabled when over cap or unchanged; Reset button.

In-app changes (`ui/src/`)

• Add `ComponentsView` + `useComponents` hook. Components is now a top-level tab next to Templates and Assets.
• Delete `SettingsPanel.tsx` and the Settings button in `TopNav`. Voice lives only in /settings (renamed Custom Instructions); Components is a tab.
• `useBrand.ts` → `useComponents.ts` (focused on the only thing the hook is still for).
• Removed dead settings-overlay/settings-panel styles.

Tests

10 new tests in `tests/test_custom_instructions.py` covering:

• Empty resource returns "" and creates no file
• Round-trip via `set_voice` → `get_voice`
• Empty content clears the file (idempotent)
• 8 KiB cap (at, over, multibyte UTF-8 — confirms byte counting, not char)
• Skill resource no longer inlines voice content

`uv run pytest tests/` → 103 passed.

Test plan

• Run `uv run pytest tests/` locally
• `cd ui && npm run build` succeeds
• Manually open `/settings/workspace/apps/synapse-collateral` in NimbleBrain — verify only Theme + Custom Instructions render; saving voice persists; agent picks up the saved instructions on next turn
• Open the in-app Components tab — verify it loads existing `components.typ`, allows editing, and saves
• Confirm in-app TopNav has no "Settings" button anymore

Related

• Pairs with Bundle data dir not workspace-scoped on installLocal (path-based install) nimblebrain#101 (orphan data-dir bug between `installLocal` and `installNamed` — platform-side fix needed before this works correctly across both install paths).