MCP `origins`: list the caller's connected origins and their repositories, and sync one, with the same rules as the API #4828

MCP now has an origins tool, placed right after outsiders, taking action (list, repos or sync) and id. The new test/mcp-origins.test.ts and the other MCP tests pass (101 tests). tsc could not check anything here: it fails with "Cannot find type definition file for 'bun'", so the forge's check is the first real type check.

• What it calls: the same functions the API routes call: listUserOrigins, listRemoteRepos and syncOriginNow, always for the caller. The answers go through the API's own originJson and remoteRepoJson, so they carry no token. I exported those two from src/http/api.ts; the routes themselves are untouched.
• Token scope: like the API's origin routes, it needs an admin token, because an origin spends the person's credential on another forge. A ro or rw token gets forbidden. list answers without id; repos and sync need id or get invalid.
• Not found: an unknown id, or another person's, answers not_found. An unreachable origin answers origin_unreachable.
• Size: the description is one short sentence, with no description on id, to keep tools/list under the 37,000-byte headroom. I did not measure the bytes the tool adds; mcp-size.test.ts passes.
• Docs and tests: docs/MCP.md has the row beside outsiders. test/mcp-origins.test.ts uses the fake origin from test/fake-origin.ts, with no network. It covers the owner listing the origin and its repositories and starting a sync, with no token in any answer. It also covers another person seeing nothing, unknown and missing ids, bad actions, and the scope refusal. test/mcp.test.ts now expects origins in the tool list. That test was out of date, not wrong.

I left out connecting and disconnecting origins and mirroring a repository over MCP. Those are separate verbs, and migrate (#4163) is where mirroring belongs.

To try it, call tools/call with { name: "origins", arguments: { action: "list" } } using an admin token.

Closes #4811

MCP now has an origins tool, placed right after outsiders, taking action (list, repos or sync) and id. The new test/mcp-origins.test.ts and the other MCP tests pass (101 tests). tsc could not check anything here: it fails with "Cannot find type definition file for 'bun'", so the forge's check is the first real type check.

• What it calls: the same functions the API routes call: listUserOrigins, listRemoteRepos and syncOriginNow, always for the caller. The answers go through the API's own originJson and remoteRepoJson, so they carry no token. I exported those two from src/http/api.ts; the routes themselves are untouched.
• Token scope: like the API's origin routes, it needs an admin token, because an origin spends the person's credential on another forge. A ro or rw token gets forbidden. list answers without id; repos and sync need id or get invalid.
• Not found: an unknown id, or another person's, answers not_found. An unreachable origin answers origin_unreachable.
• Size: the description is one short sentence, with no description on id, to keep tools/list under the 37,000-byte headroom. I did not measure the bytes the tool adds; mcp-size.test.ts passes.
• Docs and tests: docs/MCP.md has the row beside outsiders. test/mcp-origins.test.ts uses the fake origin from test/fake-origin.ts, with no network. It covers the owner listing the origin and its repositories and starting a sync, with no token in any answer. It also covers another person seeing nothing, unknown and missing ids, bad actions, and the scope refusal. test/mcp.test.ts now expects origins in the tool list. That test was out of date, not wrong.

I left out connecting and disconnecting origins and mirroring a repository over MCP. Those are separate verbs, and migrate (#4163) is where mirroring belongs.

To try it, call tools/call with { name: "origins", arguments: { action: "list" } } using an admin token.