Add change history / changelog to the project

[jangroth]

Task

Add a CHANGELOG.md at the repo root following the Keep a Changelog format.

Part 1 — Backfill

Populate the changelog by working backwards through merged PRs from today to 2026-01-01.

• Source: merged PRs only (gh pr list --state merged --base main)
• One entry per PR: title + PR number + merge date
• Group entries under date headings (per-week or per-month is fine)
• Use standard Keep a Changelog section labels where appropriate (Added, Fixed, Changed)
• Oldest entries at the bottom, newest at the top

Part 2 — Agentic upkeep

Add a GitHub Actions workflow (.github/workflows/changelog.yml) that:

• Triggers on push to main
• Reads the latest merged PR (the one that just landed)
• Prepends a new entry to CHANGELOG.md
• Commits the updated file back to main with a standard commit message

Acceptance criteria

• CHANGELOG.md exists at repo root and covers all merged PRs from 2026-01-01 onwards
• The GitHub Actions workflow runs automatically on merge and requires no manual steps
• make lint continues to pass after all changes