chore(ci): adopt monorel v1.0.0 and consolidate to one workflow

[theogravity]

Summary

Adopts monorel v1.0.0. v1 ships monorel auto, a single command that detects whether HEAD is a release-PR merge and dispatches to either the post-merge release pipeline (tag, push, publish) or the pre-merge maintenance pipeline (apply changesets, force-push staging, upsert the always-open release PR). The action wrapper no longer takes a command input; one workflow with one step does both jobs.

Changes

• Delete release-pr.yml. Its job (upsert the release PR) is now done by monorel auto on every non-release push.
• Rewrite release.yml as the single consolidated workflow. The monorel job runs the v1.0.0 action on every push to main. deploy-docs (called via workflow_call from docs.yml) gates on chore(release): so docs deploy only on the release-PR merge.
• Bump action pin from v0.14.0 to v1.0.0.
• AGENTS.md updated to describe the new single-workflow model.

Kept

• actions/setup-go@v5 (still required because GOTOOLCHAIN=local stays in monorel's offline tidy).
• The if: ... !startsWith(..., 'chore(release):') skip filter on ci.yml jobs (PR #80). The race the filter prevents (CI tidy vs release tag-push) still applies under the consolidated workflow.
• workflow_dispatch as a manual escape hatch.

Test plan

• After merge, the next non-chore(release): push to main triggers release.yml's monorel job, which upserts the always-open release PR. deploy-docs skips.
• At the next release-PR merge, the chore(release): push triggers the same monorel job, which detects the release-PR merge and tags + pushes + publishes. deploy-docs runs after the action completes and rebuilds the docs site.
• No regression in ci.yml skip behavior on chore(release): commits.

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