feat: show the base commit SHA in the PR comment's 'compared to' line

[brovatten]

The PR review comment names the base only by ref ("compared to main"), but the base branch tip moves while a PR is open (other merges, the sync baseline refresh), so it's ambiguous which main snapshot the diff was actually computed against. This appends the exact base commit's 7-char SHA as a clickable commit link — compared to main (a1b2c3d) — in all three base-reference phrasings.

Testing: Simulated the comment-body shell with sample values to confirm the rendered markdown/link, plus actionlint clean and the full unit suite green; the change is isolated to the review comment's base label.

🤖 Generated with Claude Code

[codeboarding-review]

Architecture review · no architectural changes

graph LR
    n_Orchestration_Lifecycle_Manager["Orchestration #amp; Lifecycle Manager"]
    n_Visual_Rendering_Diff_Engine["Visual Rendering #amp; Diff Engine"]
    n_User_Engagement_Integration["User Engagement #amp; Integration"]
    n_Orchestration_Lifecycle_Manager -- "Triggers structural analysis and diagram genera…" --> n_Visual_Rendering_Diff_Engine
    n_Orchestration_Lifecycle_Manager -- "Passes execution metadata and analysis status" --> n_User_Engagement_Integration
    n_Visual_Rendering_Diff_Engine -- "Provides generated Mermaid diagrams and documen…" --> n_User_Engagement_Integration
    classDef added fill:#1f883d,stroke:#0b5d23,color:#ffffff;
    classDef modified fill:#bf8700,stroke:#7d4e00,color:#ffffff;
    classDef deleted fill:#cf222e,stroke:#82071e,color:#ffffff,stroke-dasharray:5 3;

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Colors indicate component changes compared to main (290e36b): 🟩 Added · 🟨 Modified · 🟥 Removed

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_{codeboarding-action · run 27480659008}