Add release automation logic

[sydney-runkle]

Closes #470

See readme for new pattern:

1. Run `uv run release/prepare.py {VERSION}` from the root of the repository. This will:
    * Update the version number in the `pyproject.toml` files in the root and in `logfire-api`.
    * Add a new section to CHANGELOG.md with a title containing the version number tag and current date.
    * Add a line at the end of this section that looks something like [v1.0.1]: https://github.com/pydantic/logfire/compare/v{PREV_VERSION}...v1.0.1 but with the correct version number tags.
2. Curate the changes in CHANGELOG.md:
    * Make sure the markdown is valid; in particular, check text that should be in code-blocks is.
    * Mark any breaking changes with **Breaking Change:**.
    * Deduplicate the packaging entries to include only the most recent version bumps for each package.
3. Run `uv run release/push.py` from the root of the repository. This will:
    * Create a PR with the changes you made in the previous steps.
    * Add a label to the PR to indicate that it's a release PR.
    * Open a draft release on GitHub with the changes you made in the previous steps.
4. Review the PR and merge it.
5. Publish the release and wait for the CI to finish building and publishing the new version.

Sample draft release created: https://github.com/pydantic/logfire/releases/tag/untagged-f0b744b87f685d76bbd2
Sample PR (note, I've changed the base for clarity, but normally it would be against main): #511

Looks like I updated stubs as well 🤷‍♀️

[hyperlint-ai]

1 files reviewed, 1 total issue(s) found.

The style guide flagged several spelling errors that seemed like false positives. We skipped posting inline suggestions for the following words:

• gh

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (9350571) to head (b3520c8).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #504   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          132       132           
  Lines         9961      9961           
  Branches      1341      1341           
=========================================
  Hits          9961      9961           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[cloudflare-workers-and-pages]

Deploying logfire-docs with    Cloudflare Pages

Latest commit:	b3520c8
Status:	✅  Deploy successful!
Preview URL:	https://674612b8.logfire-docs.pages.dev
Branch Preview URL:	https://automate-releases.logfire-docs.pages.dev

View logs

[hyperlint-ai]

2 files reviewed, 1 total issue(s) found.

The style guide flagged several spelling errors that seemed like false positives. We skipped posting inline suggestions for the following words:

• approx_percentile_cont

[sydney-runkle]

Reversed the order here so that we can simply append in the future

[alexmojaki]

Thank you! I haven't reviewed the code because I don't think there's really a need, if there's issues we'll discover them.