feat(runners): implement Tekton Pipeline runner with native metadata discovery

Add a new TektonPipeline runner that discovers Tekton metadata natively
using a two-tier approach:
- Tier 1: HOSTNAME env var and ServiceAccount namespace file (always available)
- Tier 2: K8s API pod labels for rich tekton.dev/* metadata (best-effort)

The runner promotes 7 env vars (2 required, 5 optional) following the
Dagger runner pattern. TEKTON_TASK_NAME is optional since inline taskSpec
pipelines lack the tekton.dev/task label. TEKTON_TASKRUN_NAME falls back
to hostname derivation when K8s API labels are unavailable.

context.Context is passed as a function parameter to discoverLabelsFromKubeAPI()
rather than stored in the struct, following Go best practices.

Includes comprehensive unit tests, e2e test binary with Environment
validation, and tekton:// URI scheme for RunURI.

Signed-off-by: Vibhav Bobade <vibhav.bobde@gmail.com>
Co-Authored-By: Claude <noreply@anthropic.com>

Author: waveywaves

pkg/attestation/crafter/runner.go

@@ -96,8 +96,8 @@ var RunnerFactories = map[schemaapi.CraftingSchema_Runner_RunnerType]RunnerFacto
 	schemaapi.CraftingSchema_Runner_TEAMCITY_PIPELINE: func(_ string, _ *zerolog.Logger) SupportedRunner {
 		return runners.NewTeamCityPipeline()
 	},
-	schemaapi.CraftingSchema_Runner_TEKTON_PIPELINE: func(_ string, _ *zerolog.Logger) SupportedRunner {
-		return runners.NewTektonPipeline()
+	schemaapi.CraftingSchema_Runner_TEKTON_PIPELINE: func(_ string, logger *zerolog.Logger) SupportedRunner {
+		return runners.NewTektonPipeline(timeoutCtx, logger)
 	},
 }