docs(http): add ADR-041 MCP tool-gateway pattern for to_mcp

The to_mcp spec was describing one MCP tool per alknet operation — the
tool-bloat problem. An LLM connecting to a node with 200 operations gets
200 MCP tools dumped into its context, degrading reasoning and wasting
context budget.

ADR-041 replaces this with the tool-gateway pattern (same pattern as
opencode's memory and worktree tools): to_mcp exposes 4 fixed meta-tools
(search, schema, call, batch) that gate access to the full operation
registry. The LLM has a few tools in context, discovers operations on
demand through search + schema, then calls. Same principle as Linux's
man command — don't preload all documentation; query on demand.

Gateway tool set:
- search -> services/list (names + descriptions, AccessControl-filtered)
- schema -> services/schema (full OperationSpec for a specific op)
- call -> call.requested (Query/Mutation only, request/response)
- batch -> multiple call.requested (correlated IDs, OQ-14)

Subscription operations are excluded — MCP tool calls are
request/response by protocol design (the client blocks until
CallToolResult returns); streaming subscriptions don't fit. Subscriptions
are filtered out of search results and cannot be invoked via call.

http-mcp.md to_mcp section rewritten: the gateway tool set, Subscription
exclusion, and the service behavior (tools/list returns 4 fixed tools,
tools/call dispatches through the gateway). The 'Why' section adds the
tool-bloat rationale and the memory/worktree tool pattern that informed
the design.

README/overview ADR tables and the top-level README current-state note
updated for ADR-041.

docs/architecture/crates/http/overview.md

@@ -206,6 +206,7 @@ verified against this invariant. See ADR-014 and
 | HTTP/3 + WebTransport first-class | [ADR-038](../../decisions/038-http3-and-webtransport-as-first-class.md) | `h3` in scope, not deferred; browser streaming uses QUIC streams |
 | HTTP server + client host colocated | [ADR-039](../../decisions/039-http-server-and-client-host-colocated.md) | One crate for server + adapters (shared HTTP deps, shared mapping) |
 | WebTransport ALPN-stream-proxy | [ADR-040](../../decisions/040-webtransport-alpn-stream-proxy.md) | Browser → WebTransport stream → any ALPN handler (SSH, git, SFTP) via WASM parser |
+| `to_mcp` tool-gateway pattern | [ADR-041](../../decisions/041-mcp-tool-gateway-pattern.md) | 4 fixed gateway tools (search/schema/call/batch), not one tool per operation |
 | `alknet-call` is protocol-foundation | [ADR-003](../../decisions/003-crate-decomposition.md) Am. 1 | `alknet-http` depends on `alknet-call` (types, not peer handler) |
 | Bearer auth via `resolve_from_token` | [ADR-004](../../decisions/004-auth-as-shared-core.md) | HTTP handler credential source + resolution (settled) |
 | Stealth mode = HTTP handler on standard ALPNs | [ADR-010](../../decisions/010-alpn-router-and-endpoint.md) | Decoy for unknown paths (settled) |