Add missing attributes to Weaver registry (SCH-002, CDQ-007)

[wiggitywhitney]

Problem

The evaluation run-3 found two gaps in the Weaver semantic convention registry:

1. SCH-002: Ad-hoc attributes not in registry

Two attributes were invented by the instrumentation agent because they weren't in the registry:

• commit_story.git.subcommand — records which git CLI subcommand is being traced (show, diff-tree, rev-list, log)
• commit_story.commit.parent_count — number of parent commits (1 for regular, 2+ for merges)

Both are legitimate domain attributes that provide useful observability context.

2. CDQ-007: PII annotation on commit_story.commit.author

commit_story.commit.author stores the git author name, which is personal data per CDQ-007. Since git author names are public metadata on every commit and author attribution is core to the journal's purpose, the decision is to keep the attribute and add a note field documenting the PII acceptance.

Changes needed

In semconv/attributes.yaml:

1. Add commit_story.commit.parent_count (int) to the commit attributes group
2. Add new registry.commit_story.git attribute group with commit_story.git.subcommand (string)
3. Add PII acceptance note to commit_story.commit.author

Context

• These changes are already applied in the eval repo (commit-story-v2-eval) for run-4
• Upstream commit-story-v2 should be the source of truth for the registry
• Orbweaver issues #102 and #147 fixed the agent-side handling of schema extensions