chore(workflow): persist AI next-issue standing orders

[dipto0321]

Problem

The maintainer has to repeat the same end-to-end workflow instructions every session:

• "Start the next issue"
• "Link the issue with the PR when you create a PR"
• "Make sure to close the issue when PR resolved and successfully merge"

This is fragile: every new session re-discovers the convention.

Solution

Persist the workflow as project memory + tooling so it survives across sessions and is enforceable for both AI assistants and human contributors:

1. CLAUDE.md — add an AI workflow — standing orders section that lays out the exact sequence (start → commit → push → verify Closes linkage → squash-merge → verify auto-close → recurse). Auto-loaded by Claude Code on every session.
2. CONTRIBUTING.md — add rule 7: every PR body must reference its source issue (Closes #N or Refs #N).
3. Makefile — add make next-issue ISSUE=N and make finish-pr ISSUE=N [PR=M] targets that wrap scripts/issue-workflow.sh and add the missing pieces the script does not do (verify the linkage, use --admin, verify the issue auto-closed).

Out-of-scope

• Changing scripts/issue-workflow.sh itself — the existing 10-min gh pr checks --watch race is a separate bug.
• Auto-merging without --admin — branch protection requires a review and enforce_admins: false is intentional (allows solo-author merges), so the admin override stays.

Linked PRs

• (this PR; the three-file edit in CLAUDE.md, CONTRIBUTING.md, Makefile)