Move the pr, write-pr-description and agent-pr scripts into agentrepo pr #323

Outcome

Run the pull request transaction through agentrepo pr: draft
reading, validation, scaffold, preflight, review, gates,
confirmation, publication, drafting context and unattended mode.

The pr, write-pr-description and agent-pr skill entry scripts
forward to it and keep their frozen shell bodies as the fallback
through the N-2 compatibility window. The agent-pr scripts no
longer need the pr skill's scripts when the binary has the
behavior. The agent-pr manifest still references the pr skill's
guard-gh.sh hook until the guard cutover under #588 (owner
exact-head review), so full skill independence completes there,
after this epic.

Part of #588.

Done when

• All sixteen children are merged.
• Every behavior covered by tests/skills/pr,
  tests/skills/agent-pr and tests/skills/writers has a Go
  test or a named reason to stay, and every removed Bats case
  names a Go test and a retained frozen-path case.
• Parity with the frozen scripts and an unchanged
  mise run lint-pr-description output are recorded.
• Generated output is current.
• CI passes on macOS, Linux and Windows.
• The leak scans come back clean for this epic's own text and
  posts.
• A closing comment checks every item above against evidence.

Children

Build order follows this list. The two-digit identifiers are stable
references used in the child bodies. The validator is split in two
(02, 03), preflight in two (05, 06), publication in two (08, 09) and
forwarding in two (12, 13), because each exceeds 800 code lines as
one pull request.

• Read the pull request draft, template and prose policy in Go instead of shell #910
• Validate the draft's frontmatter and title from agentrepo pr #911
• Validate the draft's sections, comments, fences and paths from agentrepo pr #912
• Scaffold a draft from agentrepo pr #913
• Print the pull request preflight's branch state from agentrepo pr #914
• Print the pull request preflight's GitHub state and report from agentrepo pr #915
• Gather and stamp the pull request review from agentrepo pr #916
• Check the gates and resolve confirmation before publishing from agentrepo pr #917
• Push the branch and write the pull request from agentrepo pr #918
• Run agent-pr unattended from agentrepo pr #919
• Print the write-pr-description context from agentrepo pr #920
• Forward the pr skill entry scripts to agentrepo pr #921
• Forward the write-pr-description, pr check and agent-pr entry scripts to agentrepo pr #922
• Retire the pr workflow Bats suite where Go covers it #923
• Retire the compatibility, section and writer Bats suites where Go covers them #924
• Retire the agent-pr Bats suites where Go covers them #925

Children 02, 04 and 05 can proceed in parallel after 01; 14 and 15
run in parallel after their blockers, and 16 goes last because it
trims the shared helper after 14 and 15 merge.

Out of scope

Removing the frozen shell after the N-2 window: that is a later
cleanup issue under #588, and owner-tier paths go with the single
cutover issue there. Child 13 promises script-level standalone
execution only; the manifest's guard hook dependency stays until the
guard cutover.

Changes to the commit, merge and other workflow areas.

Owner

tbhb-dev/devtools