MCP `held` draws a `ui://forge/held` card: the Later gate's themes, and one button that closes the ones already done #11408

Why: the owner should be able to clear the Later gate in one tap from any harness, without reading JSON.

Done:

• [x] A read with held returns card data. CARD_OF_TOOL in src/web/studio/cards.ts maps held to a new kind held, titled "Later".
• [x] The card has one group per theme and one row per issue (#N title, linked to the issue). Rows found done on main are marked "done". The status line is the tool's says.
• [x] When approve is not empty, the card has one button, "Close N done". It calls held with apply set to approve and with the digest from the read. Its receipt is the tool's answer, and a stale read says "read it again".
• [x] docs/MCP.md's card table gains the row. A new test (test/mcp-held-card.test.ts) covers the rows, the button's arguments and that it is absent when nothing is done. bun test test/mcp-size.test.ts passes and no ceiling is raised.

Files: src/http/mcp/tools/held.ts, src/web/studio/cards.ts, docs/MCP.md, test/mcp-held-card.test.ts.

Reviewed pull #11416 at its head: ticked A read with held returns card data. CARD_OF_TOOL in src/web/studio/cards.ts maps held to a new kind held, titled "Later".; The card has one group per theme and one row per issue (#N title, linked to the issue). Rows found done on main are marked "done". The status line is the tool's says.; When approve is not empty, the card has one button, "Close N done". It calls held with apply set to approve and with the digest from the read. Its receipt is the tool's answer, and a stale read says "read it again".; docs/MCP.md's card table gains the row. A new test (test/mcp-held-card.test.ts) covers the rows, the button's arguments and that it is absent when nothing is done. bun test test/mcp-size.test.ts passes and no ceiling is raised..