ci: add release-please workflow for automated versioning and changelogs

[LeeroyHannigan]

Add Release Please workflow

What this does

Adds Release Please — a GitHub Action that automates versioning and changelog generation based on Conventional Commits.

How it works

1. When commits land on main (via merged PRs), Release Please reads the commit prefixes
2. It auto-opens a Release PR that bumps the version in Cargo.toml and generates/updates CHANGELOG.md
3. When the team merges that Release PR, it creates a GitHub Release with a git tag (e.g., v0.1.1)

Version bumping rules

Commit prefix	Bump	Example
fix:	Patch	0.1.0 → 0.1.1
feat:	Minor	0.1.0 → 0.2.0
feat!: or BREAKING CHANGE:	Major	0.1.0 → 1.0.0

What changes for contributors

Nothing, just keep using Conventional Commit prefixes (fix:, feat:, docs:, ci:, etc.) which we already do. Release Please ignores prefixes like docs:, ci:, test: for versioning purposes (they won't trigger a release on their own).

What this does NOT do

• Does not publish to crates.io or any registry
• Does not build or upload binaries (can be added later)
• Does not run on any branch other than main