MCP `feed_dismiss`: dismiss or mute a feed item, as the card's menu does #6248

POST /api/v1/feed/interactions records dismissed and muted (src/core/interactions.ts), and the home feed's ranker reads them, but no MCP tool can: an agent reading feed for a person cannot clear what they have seen.

Done:

• [ ] src/http/mcp/tools/feed_dismiss.ts: { item, kind: "dismissed"|"muted" }, writes: true, through the same validation and access check as the route (the item id pattern, an item the caller can see); move that check into one shared function both call if it lives only in src/http/feed.ts.
• [ ] After it, feed for the same caller no longer lists a dismissed item; a test proves it, and that an unknown kind or an item the caller cannot see is refused.
• [ ] docs/MCP.md lists it beside feed.

Files: src/http/mcp/tools/feed_dismiss.ts, the tool index, src/http/feed.ts if the check moves, docs/MCP.md, a test.

Parked by the steward, 2026-10-26 (owner: "get us unclogged now then resume everything else"). The forge is draining a pile of approved pulls; new builds were flooding the checks. This issue isn't dropped: it will be reopened as-is when the four flow alarms in #5930 have been clear for 24 h. The list is kept in the steward's parked-2026-10-26 file.