refactor(release): drop release-policy.json runtime dep and next branch

Strips the release-policy.json fallback from release-version.ts so the CLI
reads its version straight from packages/cli/package.json. dev → 0.0.0-private,
installed @kaelio/ktx → the real semver baked into the published package.json.
KtxCliPackageInfo collapses to { name, version, contextPackageName }; /health
no longer depends on version files surviving past a CI run.

Replaces the dual-branch (main + next) semantic-release model with a single-
branch model on main. rcs and stables interleave on the same branch via
{ name: 'main', prerelease: 'rc', channel: 'next' } / ['main']. Drops
@semantic-release/git and @semantic-release/changelog (nothing is committed
back to the repo on any channel) and the workflow's "Prepare next prerelease
branch" step plus the KTX_PRERELEASE_BRANCH plumbing. The git tag plus the
published npm artifact carry the version forward.

Updates docs/release.md, removes the two now-unused devDeps, regenerates
pnpm-lock.yaml. 611/611 @ktx/cli tests, 173/173 script tests, type-check,
biome, knip all clean.

package.json

@@ -48,10 +48,8 @@
   },
   "devDependencies": {
     "@biomejs/biome": "^2.4.15",
-    "@semantic-release/changelog": "^6.0.3",
     "@semantic-release/commit-analyzer": "^13.0.1",
     "@semantic-release/exec": "^7.1.0",
-    "@semantic-release/git": "^10.0.1",
     "@semantic-release/github": "^12.0.8",
     "@semantic-release/npm": "^13.1.5",
     "@semantic-release/release-notes-generator": "^14.1.1",