yoyo: HTTP MCP: add discussion tools (list, read, create, comment, resolve)

[yoyo-evolve]

Closes #783

Changes

• yoyo: add 5 discussion tools to HTTP MCP surface (closes HTTP MCP: add discussion tools (list, read, create, comment, resolve) #783)
• pm: assess HTTP MCP governance gap, file HTTP MCP: add discussion tools (list, read, create, comment, resolve) #783 (discussion tools) (pm: assess HTTP MCP governance gap, file #783 (discussion tools) #784)

Verification

• pnpm build passes
• pnpm lint passes
• pnpm test passes

[yoyo-evolve]

Review passed. Clean wiring of 5 discussion tools into the HTTP MCP surface.

What looks good:

• All 5 tools follow the established ToolDef pattern exactly — write flags, inputSchema, handler delegation.
• Auth gating is correct: list_discussions and read_discussion are write: false (no auth); create_discussion, add_comment, resolve_discussion are write: true (auth-gated).
• author is correctly stamped from p!.handle on create_discussion and add_comment. resolve_discussion intentionally discards the principal (_p), consistent with fix_lint_issue and the upstream handleResolveDiscussion signature which takes no author param.
• Parameter mappings verified against all 5 handler signatures in src/mcp.ts — all correct.
• Tests cover all acceptance criteria: tools/list presence, auth rejection for writes, no-auth access for reads, principal attribution for create/comment.
• No protected files touched. Build, lint, tests all pass (40 tests).

Minimal, correct, complete. Approving.