feat(mcp): Connectors — catalog + global vs per-user runtimes (§7/§14/§15)

Re-architects MCP from one owner table + agent-written registration into an
admin-curated catalog with two runtimes unioned per session, surfaced in the UI
as "Connectors" (mcp/schema stays neutral, §0.1).

Two runtimes behind one seam (§7):
- Global runtime: shared, stateless connectors (web-search, Tavily…) on the
  HOST, connected at boot from mcp_global_servers, access-filtered per user via
  mcp_global_access.
- Per-user runtime: a user's activated connectors run INSIDE their container,
  started at first login from mcp_user_servers and living until restart (§9);
  docker exec -i children die via kill_on_drop when the UserContext drops.
- McpProvider trait (mcp/provider.rs): the session round-loop never learns which
  runtime owns a server. McpManager implements it directly (inert ownerless
  bundle); UserMcpView implements global ∪ user with an accessible_global
  snapshot. Both share McpManager::connect_all; McpServerSpec +
  global_row_spec/user_row_spec turn a DB row into a connectable spec.
- mcp-client: McpServerConfig.launch_in runs a stdio command inside a container
  via docker exec -i (set at runtime, never parsed from config).

Authorization is a capability on the role, not `if role==admin` (§0.1/§14):
role_capabilities table + db/role_capabilities.rs — register_remote and
register_local_from_catalog are self-service (seeded on every new role), while
register_local_script and manage_catalog are admin-only. admin holds every
capability by construction. This removes the agent-facing register_mcp/delete_mcp
tools and the mcp kinds of list_items/toggle_item, closing the §14 RCE vector.

Schema:
- Registry: mcp_catalog (vetted templates — schema only, no live creds),
  mcp_global_servers + mcp_global_access, role_capabilities.
- Owner: mcp_user_servers (per-user activations; api_key encrypted at rest,
  catalog_name a bare TEXT snapshot, never an owner→registry FK).
- Drops the old owner table mcp_servers.

API + UI: src/frontend/api/mcp.rs (admin catalog/global/access + user
available/activate/activated, all capability-gated via require_cap);
web/components/connectors.js (<connectors-page>) renders the user view always
and the admin view for role_id === 'admin'.

Deferred: interactive per-user auth (OAuth callback / QR / SSH elicitation, §15)
— only none/api_key wired; no boot seed of catalog presets; per-(user, session)
MCP grant model still open.

crates/skald-core/src/tools/activate_tools.rs

@@ -5,7 +5,7 @@ use anyhow::Result;
 use serde_json::{Value, json};
 use sqlx::SqlitePool;
 
-use crate::mcp::McpManager;
+use crate::mcp::McpProvider;
 use crate::tools::tool_names::CONFIG_GROUP;
 use crate::tools::{Tool, ToolDescriptionLength, truncate_label, MAX_LABEL_SHORT};
 
@@ -37,7 +37,7 @@ pub struct ActivateTools {
     /// `None` for root agents (session-scoped grants).
     /// `Some(stack_id)` for sub-agents (stack-scoped grants, deleted on frame exit).
     pub stack_id:           Option<i64>,
-    pub mcp:                Arc<McpManager>,
+    pub mcp:                Arc<dyn McpProvider>,
     /// Shared in-memory grant set. Updated in-place on every call so subsequent
     /// rounds within the same turn see the new tools via `all_tool_defs()`.
     pub active_mcp_grants:  Arc<RwLock<HashSet<String>>>,