fix(roadmap-planner): drop invalid components from DORA metrics

[yhuan123]

Problem

The component dimension of DORA metrics is polluted by two classes of invalid data:

1. Unparsable version names — parseVersionName fell back to the whole version name when no component-X.Y.Z pattern matched, so legacy names like 0.3, v2.1, 1.0 each became a bogus component bucket in release_frequency / patch_ratio / lead_time.
2. v3-era plugins (plan D6, previously unimplemented) — katanomi / knative / jenkins / tekton-operator releases diluted v4 team metrics.

Fix

• parseVersionName returns an empty component instead of the whole-name fallback.
• New metrics.exclude_plugins config (implements D6); Collector.dropInvalidComponentReleases drops releases with empty or excluded components right after the existing name_regex filter, so every calculator and the versionDates maps stay clean from one place. dropExcludedComponents strips D6 plugins from issue/epic component lists (cycle_time, time_to_patch, lead_time).
• release_frequency / patch_ratio keep a defense line: component == "" is skipped instead of bucketed to "unknown".

Note: the same defense line for the Phase 2 lead_time.go rewrite ships with #197 (the calculator there is rewritten on that branch); on main, lead_time groups by issue components which are already cleaned by the collector.

Tests

• TestParseVersionName — 9 boundary cases incl. 0.3, v2.1, Sprint 2024.
• TestDropInvalidComponentReleases / TestDropExcludedComponents — both invalid classes.
• TestReleaseFrequency_SkipsEmptyComponentReleases, TestPatchRatio_SkipsEmptyComponentReleases — lock the calculator guards against silent revert.
• go test ./internal/metrics/... and go vet pass.

Accepted limitation

An issue whose only components are excluded v3 plugins still lands in the pre-existing "unknown" bucket of issue-based metrics (documented behavior in METRICS.md) rather than disappearing. Release-side metrics drop such data entirely.

Deployment note

exclude_plugins defaults to empty — production config must set it (see config.example.yaml) for the D6 exclusion to take effect.

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