docs(contributing): document PR review iteration and comment resolution workflow

[BharathASL]

Description

Ticket: None (Standalone guidelines update)

This PR formally documents the PR Review & Iteration Workflow in CONTRIBUTING.md:

• No Force-Pushing During Active Review: Prohibits amending/force-pushing on branches with active review feedback to preserve review history and range diffs.
• Separate Fix Commits: Requires subsequent review fixes to be pushed as new, standalone Conventional Commits signed off with DCO (git commit -s).
• Linking Commit SHAs: Requires responses to review comments and threads to cite the specific commit SHA(s) that introduced each fix.
• Squashing at Merge: Clarifies that branch history is squashed/rebased at merge time.

Affected Assets

• CONTRIBUTING.md

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Type of Change

• docs: Documentation improvement

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Contributor Checklist

• Commit message follows Conventional Commits format (docs(contributing): ...).
• Every commit is signed off with the Developer Certificate of Origin (git commit -s).
• Branch is rebased cleanly onto latest main with no merge commits.
• Formatting complies with .editorconfig.

[ninja6-agent]

ninja6-agent Review: Keyframe #190

• Type & Scope: Documentation update (docs/pr-review-iteration-guidelines)
• Conformance: Fully conforms to CONTRIBUTING.md standards and DCO requirements
• Standards Check: PASS (0 violations)
• CI Matrix: PASS (Build and Test, dco, standards)

Documentation additions are clear, accurate, and ready for merge.