feat(merman): support cherry-pick in GitGraph diagrams

[Robin1987China]

Issue for this PR

N/A — feat PR (feature PRs skip issue requirement per contributing guidelines)

Type of change

• New feature

What does this PR do?

Completes the GitGraph feature landed in #42179: the parser currently rejects cherry-pick with "Cherry-pick is not supported". This PR implements cherry-pick per Mermaid semantics:

• cherry-pick id: "c1" (optional tag: override) appends a cherry-picked commit to the current branch, inheriting the source commit's message and tags
• The source commit must already exist in the diagram; unknown references fail with a source diagnostic
• Cherry-picked commits use a generated id (so the commitById render map stays unambiguous) and render with a hollow ◌ glyph in a dedicated cherry cell style, echoing Mermaid's dashed-circle representation

How did you verify your code works?

• Added tests in packages/merman/src/gitgraph/diagram.test.ts: parse + inherited message/tags + tag override, missing-id and unknown-reference errors, hollow-glyph rendering
• Updated the two pre-existing assertions that expected the "not supported" error
• bun test (packages/merman) — 327 pass
• tsgo --noEmit (packages/merman) — clean

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR