Migrate release process to GitHub Actions with RC support and post-release automation

[pritt20]

Description

Replaces the hybrid CircleCI + GitHub Actions release pipeline with a fully GitHub Actions-based workflow.

Release types:

• Nightly (nightly-YYYYMMDD) — built from main for testing latest development changes, no version needed
• RC (vX.Y.Z-rc.N) — built from release-X.Y branches with
  auto-incrementing, branch validation, and approval gate
• GA (vX.Y.Z) — requires at least one RC before release

Automation:

• Automates all downstream distribution updates on GA release: godownloader script PR, Homebrew tap PR (via GoReleaser built-in), winget manifest PR, and optional Slack notification
• Uses Octo STS for short-lived tokens (Homebrew tap, winget fork) instead of long-lived PATs
• Adds Makefile targets (nightly-tag, rc-tag, release-tag) used by both CI and local workflows

Other changes:

• Switches Windows archive format from raw binary to .zip containing astro.exe (required for winget compatibility)
• Removes CircleCI configuration (.circleci/config.yml, .circleci/next_version.py)

Notion doc:

https://www.notion.so/astronomerio/Astro-CLI-Release-Process-36840290af6c80f9a9e6f5b4c6c60b97#36840290af6c80ea934bea3e1b2aa087

🎟 Issue(s)

Related #XXX

🧪 Functional Testing

List the functional testing steps to confirm this feature or fix.

📸 Screenshots

Add screenshots to illustrate the validity of these changes.

📋 Checklist

• Rebased from the main (or release if patching) branch (before testing)
• Ran make test before taking out of draft
• Ran make lint before taking out of draft
• Added/updated applicable tests
• Tested against Astro-API (if necessary).
• Tested against Houston-API and Astronomer (if necessary).
• Communicated to/tagged owners of respective clients potentially impacted by these changes.
• Updated any related documentation

[coveralls-official]

Coverage Report for CI Build 26297149101

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage decreased (-0.1%) to 39.653%

Details

• Coverage decreased (-0.1%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 1885 coverage regressions across 7 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

1885 previously-covered lines in 7 files lost coverage.

File	Lines Losing Coverage	Coverage
airflow/mocks/DockerCLIClient.go	722	0.0%
airflow/mocks/DockerRegistryAPI.go	683	0.0%
airflow/mocks/DockerComposeAPI.go	176	0.0%
airflow/docker.go	152	77.12%
cloud/deploy/deploy.go	82	81.15%
airflow/standalone.go	38	71.89%
cmd/otto.go	32	19.35%

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Coverage Stats

Relevant Lines:	66078
Covered Lines:	26202
Line Coverage:	39.65%
Coverage Strength:	9.54 hits per line

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💛 - Coveralls