feat(site): group selected MCP servers into one composer pill - #29258

When more than one MCP server is active in the Agents chat composer, the individual server pills now collapse into a single "N MCP servers" pill with an overlapping icon stack. Clicking it opens a popover that lists each server with its own remove control; force_on servers show a lock (with visually hidden "Always on" text) instead. A single active server keeps the existing named pill, and servers that still need OAuth stay excluded from the group as before.

The settings-page MCPServerIcon moved to site/src/modules/mcpServers/MCPServerIcon.tsx with a tile (settings) and circle (composer and picker) variant so there is one icon renderer, and it is decorative (alt="") because every consumer renders the server name next to it or labels the containing control. The overlapping icon stack moved out of MCPServerPicker.tsx into MCPServerIconStack.tsx so the picker and the composer share it.

Stories cover the closed, open, disabled, five-server, and nested-overflow states; the overflow stories that relied on many MCP pills now use a long attached workspace name to force +N. Vitest covers the remove callbacks, the single-pill path, the OAuth-pending exclusion, and the disabled state.

Known limitation, consistent with the existing pills: removing the second-to-last server from the group unmounts the group pill together with its popover, so keyboard focus returns to the document rather than to a surviving control, the same as dismissing any existing badge with its X. Restoring focus consistently across all badges is left for a follow-up.

Validation

• pnpm check, pnpm lint:types, pnpm lint:compiler, pnpm lint:circular-deps, knip, and the unit and Storybook tests for the touched files (AgentsPage composer and picker, MCPServersPage) pass.
• Pre-commit and pre-push hooks passed on every commit. The pre-push runs used an LD_PRELOAD shim that makes connect() to loopback:3000 fail with ECONNREFUSED, matching CI where nothing listens on jsdom's default origin; without it a service on :3000 on the development host answers unmocked requests and crashes unrelated settings-page tests. The hook itself ran unmodified.
• Dogfood UAT ran through a remote Coder Agent against a running deployment built from f59e7d37c58 and passed: grouped pill, popover removal 3 to 2 to 1 to 0, force_on lock, OAuth-pending exclusion, 414px nested overflow, existing-chat persistence, and keyboard navigation. The later commits (89427f1f7a3, dcdc6267ce1, 40ae39ef964) only change the icon component's home and alt text, add the hidden lock text, and adjust tests; they were not re-exercised remotely.
• Codex review round 1 (head 89427f1f7a3): one finding refuted by a Chromium probe (nested removal keeps the overflow popover open), one declined as a pre-existing cross-badge focus behavior (see above), two fixed in dcdc6267ce1 (shared icon component, removed presence-only test). Round 2 (head dcdc6267ce1): two findings fixed in 40ae39ef964 (hidden "Always on" text for locked rows, removed the remaining DOM-absence test assertions).

Xum (Coder's coding agent) implemented this on behalf of @ibetitsmike.