feat(discovery): DaemonSet discovery with scheduling predicates

[diranged]

Summary

• Implements Phase 2 (DaemonSet Discovery) of the node readiness controller
• Uses upstream k8s.io/component-helpers scheduling predicates to evaluate nodeSelector, nodeAffinity, and tolerations against each node
• Strips configured startup taints before toleration evaluation to determine steady-state DaemonSet scheduling
• Adds configurable exclusions by namespace/name and by label selector (self-service opt-out)
• Wires discovery into the reconciler with metrics instrumentation

Changes

• internal/discovery/discovery.go — Core discovery algorithm with ExpectedDaemonSets(), startup taint stripping, synthetic pod construction, and exclusion logic
• internal/discovery/discovery_test.go — 16 unit tests covering scheduling predicates, taint stripping, exclusions, and edge cases
• pkg/config/config.go — Added ByLabel exclusion support with LabelSelector and LabelSelectorRequirement types
• internal/controller/node_readiness_controller.go — Wired in discovery, logs discovered DaemonSets, emits per-node gauge metric
• internal/controller/node_readiness_controller_test.go — Updated tests for new Discovery field
• cmd/main.go — Creates discovery instance and passes to reconciler

Test plan

• All 16 discovery unit tests pass locally
• All controller unit tests pass locally
• E2E tests pass locally (KIND cluster)
• go vet clean
• CI passes

Closes #6

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