docs: correct the release docs to match what the workflows now do

[StoneyJackson]

Five separate inaccuracies, all of which would mislead a maintainer:

• Both README and the maintainer guide claimed feat!: produces a major. It does not. semantic-release's Angular preset ignores the ! shorthand, so feat!: alone yields no release while CI stays green. The BREAKING CHANGE footer is what does the work, which also makes merge commits safer than squash for a major.
• The release flow omitted the plan job and described build as unconditional. Builds now happen only when a release is actually coming.
• The weekly bump was described as touching only the two image configs and explicitly "not" the feature. It updates five files, bumps the feature's own version, derives the release type from plcc-ng's major, and retags image references on a major.
• Bootstrap said to require "the CI status check". Requiring job names is what hung PRs in both repos today; require ci-gate and only ci-gate.
• Housekeeping implied candidates accumulate on every merge; infra-only merges no longer build at all.

Neither commit type releases, so this changes nothing about versioning.

[Copilot]

Pull request overview

Updates the repository’s release documentation to reflect the current GitHub Actions workflows and semantic-release behavior, reducing the risk of maintainers following outdated or misleading instructions.

Changes:

• Corrects Conventional Commit guidance for major releases to rely on BREAKING CHANGE: footers (not feat!:).
• Documents the Release workflow’s plan step and that image builds/publishing are conditional on an actual release.
• Expands the “weekly plcc-ng bump” and bootstrap/ruleset guidance to match current automation (including ci-gate).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
README.md	Updates contributor-facing Conventional Commit / major-release guidance to match semantic-release behavior.
docs/maintainer-guide.md	Aligns maintainer release flow, weekly bump process, and branch protection guidance with current workflows (plan/build gating, ci-gate, housekeeping).

Comments suppressed due to low confidence (1)

docs/maintainer-guide.md:152

• This section says infra-only merges “skip the build entirely”, but earlier the guide correctly explains that the build job still runs and only its steps are skipped when plan says no release is coming. Consider rewording here to avoid contradicting the earlier explanation.

  the package settings UI if clutter bothers you. Candidates are only
  produced for commits that actually release — infra-only merges skip the
  build entirely (see `plan`, above), so they no longer leave orphans.

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