Move composite action release-note collection out of action.yml into tested code

[lbliii]

Context

action.yml has grown into an 846-line composite action with embedded bash, inline Python, GitHub API calls, release-note collection, rendering, posting, and asset upload logic. This makes behavior hard to test outside Actions and makes review of release/report changes noisy.

Evidence

• wc -l action.yml reports 846 lines.
• The Collect GitHub PR data step embeds a large Python script inside YAML.
• That script handles PR queries, release-note markers, linked issues, dependency parsing, direct commits, diff stats, benchmark placeholders, and output JSON.
• Tests cover report templates and parsers, but this collector logic is not easy to unit test as a normal Python module.

Proposed Scope

• Extract release-note/PR collection into a Python module or script under src/kida or scripts/ with focused tests.
• Keep action.yml as wiring: setup, call collector, render, post.
• Add fixtures for release-note markers, dependency PRs, direct commits, missing tags, and malformed GitHub responses.

Done When

• action.yml is materially smaller and mostly declarative.
• Collector behavior has local tests that do not require a live GitHub event.
• Existing release-note action workflows continue to work.