Simplify the agent host, the world and the gateway

[SaladDay]

B4 simplicity audit, lane L4: the agent host, the world and the gateway. Net −143 production lines.

Defect fixed: the gateway stops before a view's end is reported. agenthost/doc.go promises that the gateway has stopped before the Process reports the view's end. Before this change, ownedView.end cancelled a context and moved on, while the gateway closed in context.AfterFunc callbacks. Now:

• gateway.Start returns a synchronous stop, and end waits for it.
• One connSet tracks every connection in both directions: accepted, dialed upstream, HTTP/2, upgraded MCP and CONNECT tunnels. A connection leaves the set only after it has closed. Adding to a closed set resets the connection.
• Stop cancels, closes the listeners, waits for the accept loops, then resets every connection.
• TestStopClosesEveryConnection failed 4 of 20 runs against the previous gateway; it passes every run now.

Deletions:

• gateway:
  • one model provider and one endpoint, replacing the named model list, its uniqueness check and endpoint map, Endpoints.Placeholder and SessionNetwork;
  • CloseIdleConnections and the transport list;
  • the test's private FUSE world, replaced by fileservicetest plus worldfs.
• agenthost:
  • the Error struct, replaced by %w wrapping with the same sentinels;
  • the attachLink interface;
  • processTable/procfs, replaced by an injectable task-listing function;
  • the provider's JSON round trip in handoff.
• worldfs:
  • the Error struct;
  • the hand-written no-op and ENOSYS methods, replaced by an embedded fuse.NewDefaultRawFileSystem(), with errno assertions.
• agent: the AST test that forced View: nil.
• agent/mcode: config.yaml, workspace-profile.json and mcp.json are built in memory and written once each. The bytes match the previous output in 7 modes.

Test-only fix: two worldfs tests hung on one CPU, because the test process both serves the FUSE world and touches it. They now pre-poll go-fuse's epoll file and fault a mapping in with MADV_POPULATE_READ. Production never opens world files in its own process.

Testing:

• The gateway, agenthost and worldfs privileged suites on all CPUs and with --cpuset-cpus=0.
• The stop, listener, relay and view-end tests at -count=50.
• Gateway -race -count=50.
• Unprivileged tests, vet, and the darwin and windows builds.

Blind review (Codex) asked for the complete stop; this PR includes it.

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