desktop: no per-agent MCP injection channel for preset/custom ACP runtimes (omp, hermes)

When a desktop agent runs through a tier-2 preset (omp, hermes) or a custom harness, the desktop app never passes an MCP server to the runtime: `BUZZ_ACP_MCP_COMMAND` is derived only from `known_acp_runtime` (goose/claude/codex/buzz-agent), so it is always empty for omp/hermes/custom runtimes, and there is no other injection channel. The same MCP config works fine in omp/goose/codex/claude-code/opencode directly: ACP v1 natively defines stdio/http/sse MCP servers (agentclientprotocol/agent-client-protocol, schema/v1/schema.json), all accept client-injected mcpServers, and omp declares http+sse capability in initialize — so this is a Buzz-specific gap.

**Use case**: A user runs open-connector (streamable HTTP MCP with Bearer auth) against Notion. It works in omp's interactive sessions, but cannot reach a Buzz agent running on omp — we had to bridge stdio→HTTP ourselves.

**Evidence (HEAD 857e63c)**:
- `desktop/src-tauri/src/managed_agents/runtime.rs:527-529` (+ 294-298, 466-469): MCP command derived only via `known_acp_runtime(effective_command)`; comment: "pure derivations of the command… rather than the frozen record snapshot"
- `desktop/src-tauri/src/commands/agent_models.rs:884-888`: "the per-record field is never read by the runtime"; `types.rs:258-262` "Never read at spawn"; `runtime_metadata.rs:9-10` "Vestigial"
- `desktop/src-tauri/src/managed_agents/custom_harnesses.rs:49-75`: `HarnessDefinition` has no `mcp_command`; definition env filtered against `RESERVED_ENV_KEYS` (`env_vars.rs:73` includes `BUZZ_ACP_MCP_COMMAND`), and `validate_user_env_keys` rejects on save
- `discovery/presets.rs:104-112`: omp is a tier-2 preset, but `KNOWN_ACP_RUNTIMES` (discovery.rs:75-180) has only goose/claude/codex/buzz-agent → `known_acp_runtime("omp")` = None
- `crates/buzz-agent/src/lib.rs:302-310`: initialize declares `mcpCapabilities {http:false, sse:false}`; `mcp.rs` only rmcp `TokioChildProcess` (stdio)
- `crates/buzz-acp/src/config.rs:261` → `lib.rs:4280`: single stdio server from `BUZZ_ACP_MCP_COMMAND`

**Proposal**:
1. Wire the structured MCP config (#4164's `BUZZ_ACP_MCP_CONFIG`) into the desktop spawn path, for preset and custom runtimes alike. When `BUZZ_ACP_MCP_CONFIG` is absent, keep the existing `known_acp_runtime`-derived command as the fallback so default behavior is unchanged. The managed-agents reconciler (`migration.rs:975/1220`) must recognize the new config source, or startup will rewrite `managed-agents.json` from the directory table and clobber the new channel.
2. When #4271 (Streamable HTTP) lands, forward `http` mcpServers to runtimes that declare the capability (buzz-agent would need to declare it; rmcp already supports streamable HTTP).
3. Credential isolation per #4154: third-party MCP servers must not receive `BUZZ_PRIVATE_KEY`/`BUZZ_AUTH_TAG`.

**Related**: #2899 #2900 #3115 #3399 #4154 #4164 #4271 #4301 #4480 #4515