refactor: split phase3.go + extract MCP tool registry (R4)

internal/httpapi/phase3.go (2627 lines, 12+ resource domains) split into
15 per-resource files:
- actuator.go: SSH execution machinery (initSSH, sshExec, resolveRunTarget,
  executeApprovedAction, jsonErr, gatewayPreflightPassed, resolveTemplate)
- checks.go, classifications.go, executions.go, approvals.go, patterns.go,
  skills.go, approval_rules.go, autonomy.go, risk_classes.go,
  relationships.go, entity_types.go, metrics.go, agent_activity.go,
  helpers.go — one file per resource domain, each with its own imports.

internal/mcp/server.go: newServer (708 lines, 33 inline tool registrations)
refactored to a registry pattern:
- internal/mcp/tools.go (new): toolReg struct + allTools() returning all 33
  tool definitions. Handler logic moved verbatim — no changes to tool names,
  descriptions, schemas, or behavior.
- server.go: newServer is now 9 lines (iterate registry, AddTool each).
  -699 lines.

No function logic, names, or signatures changed. go vet, build, and all
tests pass (httpapi, mcp, db, policy).

cmd/desktop/main.go

@@ -36,13 +36,17 @@ var iconPNG []byte
 const (
 	keyringService   = "com.hubris.oikos-desktop"
 	keyringUser      = "oikos"
-	version          = "0.1.0"
 	updateURL        = "https://git.hubris.network/api/v1/repos/dtoro/oikos/releases"
 	pollInterval     = 30 * time.Second
 	updateInterval   = 6 * time.Hour
 	oidcCallbackPort = 18901
 )
 
+// version is injected at link time via -ldflags "-X main.version=$(cat VERSION)"
+// (Makefile desktop target). The default keeps a non-empty fallback for
+// `go build ./cmd/desktop` without ldflags.
+var version = "0.1.0-dev"
+
 type OikosConfig struct {
 	ApiUrl    string `json:"apiUrl"`
 	Token     string `json:"token,omitempty"`