ci: replace manual versioning with release-please

[milesangelo]

Summary

Replaces the manual <Version> + tag-driven publish pipeline with release-please on Conventional Commits. The package version becomes managed entirely by release-please via .release-please-manifest.json, and a single release-please.yml workflow opens Release PRs and pushes to NuGet.org when those PRs are merged.

• Single source of truth: .release-please-manifest.json (seeded 0.0.0). The csproj has no <Version> or <PackageReleaseNotes>.
• Single release workflow: .github/workflows/release-please.yml runs on every push to main, opens/updates a Release PR, and on merge tags + publishes via dotnet pack /p:Version=$VERSION → dotnet nuget push.
• Trunk-only: develop branch and v* tag triggers are gone. ci.yml is now a pure build/test/format/analyzers/security workflow on main only.
• Legacy workflows deleted: build.yml and publish-nuget.yml (both superseded).
• Contributor docs updated: CONTRIBUTING.md documents the Conventional Commits requirement; CLAUDE.md describes the new release flow; the bug template's version placeholder is refreshed.

File changes

File	Action
release-please-config.json	new
.release-please-manifest.json	new (seeded 0.0.0)
CHANGELOG.md	new (stub; release-please owns it)
.github/workflows/release-please.yml	new
.github/workflows/ci.yml	strip publish jobs + tag/develop triggers
.github/workflows/build.yml	delete
.github/workflows/publish-nuget.yml	delete
Coinbase.AdvancedTrade.Client/Coinbase.AdvancedTrade.Client.csproj	remove <Version> and <PackageReleaseNotes>
CONTRIBUTING.md	add ## Commit Messages section
CLAUDE.md	rewrite ## CI/CD and ## Releases
.github/ISSUE_TEMPLATE/bug_report.md	refresh version placeholder
.gitignore	ignore docs/superpowers/ (planning artifacts)

How the new release flow works

1. Developer commits to main using Conventional Commits (feat:, fix:, feat!:, etc.).
2. release-please.yml opens or updates a chore(main): release X.Y.Z PR with the next version and a changelog.
3. Maintainer reviews and merges the Release PR.
4. release-please creates the vX.Y.Z tag and a GitHub Release; the same workflow's publish job builds with /p:Version=X.Y.Z, packs, and pushes to NuGet.org.
5. Pre-1.0 (current state): feat: → patch bump, feat!: → minor bump. To cut 1.0.0, push a commit with footer Release-As: 1.0.0.

Maintainer post-merge checklist

These are one-time, destructive operations the maintainer needs to do after this PR merges (they are not automated):

• Delete stale tags locally and from the remote:

  git tag -d v0.0.0 v0.1.0-alpha v0.1.1-alpha
  git push origin :refs/tags/v0.0.0 :refs/tags/v0.1.0-alpha :refs/tags/v0.1.1-alpha

• Delete the develop branch (local + remote) if present:

  git push origin --delete develop

• Update branch protection rules on main — remove any required checks that reference the deleted workflow names (Build and Test, Publish to NuGet.org). PRs will be unmergeable until these are removed.
• Confirm NUGET_API_KEY secret is still present and valid in GitHub Actions.
• Bootstrap the first release. None of the commits in this PR use a release-triggering type (all chore: / ci: / docs:), so release-please will not open a Release PR until a real feat: or fix: lands. Either:
  • Squash-merge this PR with title feat: replace manual versioning with release-please (will trigger an immediate 0.0.1 Release PR), OR
  • Merge with the current title and push a separate fix: or feat: commit afterward to bootstrap the flow.

Test plan

• dotnet build -c Release — clean (0 errors, 0 warnings)
• dotnet test -c Release — 116/116 passing (102 unit + 14 integration)
• dotnet format --verify-no-changes — exit 0
• All workflow YAML files validate
• release-please-config.json and .release-please-manifest.json validate
• dotnet pack ... /p:Version=0.0.1 produces Coinbase.AdvancedTrade.Client.0.0.1.nupkg (verified locally — confirms the CI publish step's version injection works)
• After merge: confirm release-please.yml runs successfully on the merge commit
• After bootstrap commit: confirm release-please opens a Release PR
• After merging the Release PR: confirm publish job tags, releases, and pushes to NuGet.org

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