feat: MCP ping tool, tightened descriptions, and Hermes client docs

- Add  MCP tool — lightweight connectivity check returning server
  identity, no DB hit (resolves agent connection-test friction)
- Tighten 6 tool descriptions (get_relations, get_health_summary,
  query_metrics, get_trend, get_event_timeline, ping) to be searchable
  in the first 8-12 words
- Document Hermes MCP client setup in ADR-0012 with token security caveat
- Move completed plan to plans/done/

internal/mcp/tools.go

@@ -30,6 +30,12 @@ type toolReg struct {
 // newServer registrations.
 func allTools(pool *db.Pool, agentID uuid.UUID) []toolReg {
 	return []toolReg{
+		{tool: &mcp.Tool{Name: "ping", Description: "Lightweight connectivity check. Returns server identity, no DB hit.",
+			InputSchema: objSchema(),
+		}, handler: func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
+			return textResult(`{"ok":true,"server":"oikos","version":"dev"}`), nil
+		}},
+
 		{tool: &mcp.Tool{Name: "get_entity", Description: "Get an entity by slug or UUID",
 			InputSchema: objSchema(prop{"slug_or_id", "string", "Entity slug (e.g. host:hubris) or UUID"}),
 		}, handler: func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
@@ -57,7 +63,7 @@ func allTools(pool *db.Pool, agentID uuid.UUID) []toolReg {
 				nStr(args["type"]), nStr(args["state"]), nStr(args["q"]), limit), "entity_table"), nil
 		}},
 
-		{tool: &mcp.Tool{Name: "get_relations", Description: "Get relationships for an entity",
+		{tool: &mcp.Tool{Name: "get_relations", Description: "List inbound/outbound edges for one entity",
 			InputSchema: objSchema(prop{"entity_id", "string", "Entity slug"}),
 		}, handler: func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 			args := argsMap(req)
@@ -84,7 +90,7 @@ func allTools(pool *db.Pool, agentID uuid.UUID) []toolReg {
 				slug, depth), nil
 		}},
 
-		{tool: &mcp.Tool{Name: "get_health_summary", Description: "Current fleet health summary",
+		{tool: &mcp.Tool{Name: "get_health_summary", Description: "Full fleet health per entity — healthy/degraded/down/unknown",
 			InputSchema: objSchema(),
 		}, handler: func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 			return queryRows(ctx, pool, `
@@ -446,7 +452,7 @@ func allTools(pool *db.Pool, agentID uuid.UUID) []toolReg {
 			return textResult(fmt.Sprintf("Ended: %s —%s→ %s.", source, relType, target)), nil
 		}},
 
-		{tool: &mcp.Tool{Name: "query_metrics", Description: "Query time-series metrics",
+		{tool: &mcp.Tool{Name: "query_metrics", Description: "Time-series metrics with bucketed avg/min/max over N hours",
 			InputSchema: objSchema(prop{"hours", "integer", "Look-back window in hours (default 24)"}),
 		}, handler: func(ctx context.Context, req *mcp.CallToolRequest) (*mcp.CallToolResult, error) {
 			args := argsMap(req)
@@ -628,7 +634,7 @@ func allTools(pool *db.Pool, agentID uuid.UUID) []toolReg {
 			WHERE e.entity_id = $1`, eid), nil
 		}},
 
-		{tool: &mcp.Tool{Name: "get_trend", Description: "Get metric trends for an entity",
+		{tool: &mcp.Tool{Name: "get_trend", Description: "Metric slope, variance, and averages for an entity over N days",
 			InputSchema: objSchema(
 				prop{"entity_id", "string", "Entity slug"},
 				prop{"days", "integer", "Look-back window in days (default 7)"}),
@@ -649,7 +655,7 @@ func allTools(pool *db.Pool, agentID uuid.UUID) []toolReg {
 			ORDER BY metric`, slug, days), nil
 		}},
 
-		{tool: &mcp.Tool{Name: "get_event_timeline", Description: "Get recent events",
+		{tool: &mcp.Tool{Name: "get_event_timeline", Description: "Recent events filtered by severity and entity slug",
 			InputSchema: objSchema(
 				prop{"severity", "string", "Filter by severity (info, warn, error)"},
 				prop{"entity_slug", "string", "Filter by entity slug"},