refactor(control-plane): split handlers/ui/dashboard.go into cache/format/aggregate files

[kiranannadatha8]

Closes #417

Summary

Splits control-plane/internal/handlers/ui/dashboard.go (1493 LOC, 29 funcs, 20+ types, 4 mixed concerns) into four focused files within the same handlers/ui package. Pure file moves: no renames, no behavior change, no exported-API change.

File layout

File	LOC	Contents
dashboard.go	348	DashboardHandler + constructor; base response types (DashboardSummaryResponse, AgentsSummary, ExecutionsSummary, PackagesSummary); both public handlers (GetDashboardSummaryHandler, GetEnhancedDashboardSummaryHandler); request-parsing helpers (parseTimeRangeParams, calculateComparisonPeriod)
dashboard_cache.go	146	DashboardCache + 3 methods; TimeRangePreset type + 5 constants; EnhancedDashboardCache + enhancedCacheEntry + 3 methods; getTTLForPreset, generateCacheKey
dashboard_format.go	177	Response-shape type declarations only (EnhancedDashboardResponse, EnhancedOverview, ExecutionTrends, AgentHealthSummary, WorkflowInsights, IncidentItem, heatmap/hotspot types, etc.)
dashboard_aggregate.go	839	All build* aggregators (10); get*Summary getters (3); helpers calculateSuccessRate, computeMedian, maxTime

Before: 1 file × 1493 LOC → After: 4 files × {348, 146, 177, 839} LOC (sum 1510; +17 lines from 3 extra package + import blocks).

Why this split

The issue proposed dashboard.go / dashboard_aggregate.go / dashboard_format.go / dashboard_cache.go. The existing code has no literal format* helpers — response shaping lives in struct tags — so dashboard_format.go holds the response-shape types, which is the spirit of the proposal (a file dedicated to response shape, separate from handlers and aggregators). Happy to fold the types back into dashboard.go if the reviewer prefers; the diff is tiny either way.

Guardrails respected

• No renames (every symbol preserved — diff <(grep '^(func|type) ' original | sort) <(grep '^(func|type) ' new/* | sort) is empty)
• No behavior change (byte-identical function bodies — verified with ranged diff against HEAD:dashboard.go)
• No caller changes needed (server/server.go:1249-1251 route registration + all handlers/ui/*_test.go files reference exported symbols by name; same-package visibility preserved)
• Scope limited to the four dashboard files

Test plan

• cd control-plane && go test ./internal/handlers/ui/... -count=1 → ok in 6.038s (primary gate from the issue)
• cd control-plane && go build ./... → green
• cd control-plane && go vet ./internal/handlers/ui/... → clean
• gofmt -l control-plane/internal/handlers/ui/dashboard*.go → clean
• cd control-plane && go test ./... -count=1 → everything passes except two pre-existing failures in internal/server (TestGenerateAgentFieldServerIDFallsBackWhenGetwdFails, TestStartAdminGRPCServerReturnsListenError) which also fail on a fresh clone of main — environment-specific, unrelated to this PR
• CI green

Note on -race: go test ./internal/handlers/ui/... -race trips checkptr inside github.com/boltdb/bolt@v1.3.1 (an unsafe pointer cast in the dep) under Go 1.26. This also reproduces on upstream main and is outside this PR's scope.

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