chore(governance): retire the develop branch

[L4XB]

Why

With one maintainer there is nothing to integrate. Every change already goes
through a pull request that main gates on 19 required checks, code scanning,
DCO, CLA, signed commits and linear history. A second long-lived branch adds no
safety on top of that — it only has to be kept in step by hand.

It was not free. Because both branches allow squash merges only, the same
content necessarily gets a different commit on each. "Are they in sync?" could
only be answered by comparing tree hashes, and it looked like drift when it was
not. Keeping them level cost two pull requests (#95, #100) and two full CI runs
that produced no product change. With strict up-to-date on both, every merge
on one side invalidated pending work on the other.

Nothing depends on it. main is the default branch, no workflow needs
develop to exist, and release.yml already refuses to run unless it is
dispatched from refs/heads/main with the tag contained there — v0.2.0-alpha.1
was cut directly from main, even though the runbook said to promote from
develop.

What changes

ci.yml, codeql.yml, security.yml, dependency-review.yml	branches: [main, develop] → [main]
.github/dependabot.yml	8 target-branch: develop entries removed
CONTRIBUTING.md, GOVERNANCE.md	branch from and target main; the governance text says why there is no integration branch
docs/RELEASING.md, docs/RELEASE_CHECKLIST.md	releases are cut from reviewed main commits
.github/scripts/claim_issue.py + its test	the reply /claim sends a new contributor

After this: grep -rn '\bdevelop\b' across the tree returns nothing outside
development-dependencies and the word "developer".

Verification

uv run pytest tests/test_repository_automation.py -q  →  72 passed
uv run ruff check .github/scripts tests               →  All checks passed!
uv run ruff format --check .github/scripts tests      →  24 files already formatted
dependabot.yml: 8 ecosystems intact, 0 target-branch entries

After this merges

Two steps that are not part of the diff, in this order:

1. Delete the Protect develop ruleset (23079701).
2. Delete the develop branch.

The six Dependabot pull requests that targeted develop were landed first in
#111, so nothing is orphaned.

[L4XB]

I have read and agree to the SixSentences CLA v1.0.