feat(control-plane): add authoritative native MCP fleet inventory (#949)

Every pre-existing runtime surface is per-process. gitea_get_runtime_context
and gitea_assess_master_parity describe only the server answering the call,
and gitea_assess_mcp_namespace_health accepts process, probe_result and
registered_tools from the caller, so it cannot constrain the caller. The
control-plane sessions table records allocator task sessions, not server
processes. Five self-reports of one revision therefore never proved that five
processes exist, that no sixth exists, or that all five share one client
cohort.

Add gitea_assess_fleet_inventory, a strictly read-only capability that takes
no evidence parameters. It combines two sources that must agree:

- a control-plane runtime registry row each server writes about itself,
  from the official entrypoint, immediately after the native transport bind;
- a process observation the answering server performs, never the caller.

A member is running only when both agree and the observed process predates
its registration, so configuration alone never counts and a recycled PID
cannot impersonate an exited server. Classification is a pure function of the
snapshot, so gitea-controller and gitea-reconciler return the same verdict.

Missing, duplicate, unexpected, stale and unregistered members are reported
in separate fields rather than collapsed into one error, because each needs a
different operator action. single_cohort is derived only from recorded cohort
identity and stays null when unknown, so matching revisions never establish a
cohort. Any unreadable registry, unavailable listing, unregistered process,
unknown cohort or unknown revision sets inventory_complete and
mutation_gate_satisfied false with a specific blocked_reason.

The capability performs no restart, reconnect, drain, lease mutation, issue
mutation or process termination, never terminates a duplicate, and never
manufactures restart evidence. The only signal sent is signal 0.

Also resolves the fleet namespace from the expected roster:
role_namespace_gate.infer_mcp_namespace recognises only author and reviewer
and echoes the profile name otherwise, which would have mislabelled the
controller, merger and reconciler members. Widening that shared helper is
controller role-metadata work owned by #950 and is deliberately not done here.

Schema v5 to v6 is additive and idempotent: one new table, no existing table,
tool signature or result field changed. Two control-plane tests that pinned
the schema version to the literal 5 now assert against SCHEMA_VERSION.

#950 (controller role metadata), #951 (restart receipts) and #952 (stale-lease
consistency) remain separate and untouched.

Tests: 118 new across tests/test_mcp_fleet_inventory.py,
tests/test_control_plane_db_server_runtimes.py and
tests/test_fleet_inventory_tool.py, covering every acceptance criterion
including the multi-LLM duplicate-server regression that motivated the issue.

Closes #949

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01NBaQKcRRKeKbZuhk72MhEf

docs/mcp-tool-inventory.md

@@ -54,6 +54,7 @@ that gates each call, not which tools exist.
 - `gitea_assess_already_landed_reconciliation`
 - `gitea_assess_conflict_fix_classification`
 - `gitea_assess_conflict_fix_push`
+- `gitea_assess_fleet_inventory`
 - `gitea_assess_gitea_operation_path`
 - `gitea_assess_master_parity`
 - `gitea_assess_mcp_namespace_health`