fix: prevent PR titles with special characters from breaking Slack changelog workflow

[BenjaminLangenakenSF]

Description

PR titles containing double quotes and unquoted parentheses — e.g. BE-14720: Fix French typo "à été organisé(e)" in electronic meeting text — break the Get PR Details step with a bash syntax error, so the changelog job fails and no Slack message gets posted at all.

More generally, this workflow spliced ${{ github context values }} directly into shell script text across all three steps. Since GitHub Actions substitutes ${{ }} as literal text before bash parses the line, any PR title containing backticks or $(...) could trigger arbitrary command execution in the Post to Slack step (PR titles are attacker/contributor controlled).

This PR fixes it by passing every GitHub context value through an env: block instead, so bash treats it as an opaque variable value rather than re-parsing it as script text. Also fixed a related latent bug: the JSON-escaping sed handled " and newlines but not \, so a title containing a backslash would have produced invalid JSON.

Verified locally: the original line reproduces syntax error near unexpected token '(' with the BE-14720 title; the same title passed through an env var (this PR's approach) round-trips cleanly.

Fixes # (no tracked issue — found while investigating why BE-14720's title would affect the changelog Slack post)

Type of change

• Bug fix
• New feature
• Breaking change

Checklist

• README updated (if needed)
• Version updated (if needed)
• Documentation updated (if needed)

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