Add duplicate-check step to create-issue skill

[jeffhorn-nava]

What

The create-issue skill had no step to catch a duplicate before filing, so a new issue duplicating an open one passed straight through unless a human happened to ask.

Closes #19

How

A new Step 1, "Check for duplicates," runs ahead of template selection: search all issues by keyword with gh issue list --state all --search, judge by whether the same work is already tracked rather than a shared topic, and handle three outcomes: comment on or update an open near-duplicate instead of filing; note in the body why a still-warranted new issue is not a duplicate; or get explicit confirmation from the person at the keyboard before filing over a plausible duplicate. Closing an issue as a duplicate applies the existing duplicate label. The remaining steps renumber 2 through 6. The phase-listing pointers in AGENTS.md and CONTRIBUTING.md and the skill's frontmatter description are synced to include the new step.

Test plan

• Walked the skill end to end against the issue templates; the steps read cleanly 1 through 6.
• Confirmed the duplicate label already exists in the repo.
• Mapped each definition-of-done item in Add a duplicate-check step to the create-issue skill #19 to specific lines in the new step.

Notes for reviewers

The create-pr counterpart of this gate, raised in an issue comment, is deliberately left to #27. No runtime surface changed, so no tests were added.

[jeffhorn-nava]

Code review

Found 1 issue:

1. The new "Check for duplicates" step claims gh issue list --state all surfaces PRs, but it does not: gh issue list returns only issues regardless of --state. Verified against this repo, --state all returns issue numbers only and none of the open/merged PR numbers. Since the step's stated goal is "a PR may already be doing the work," an author following it verbatim never actually checks PRs and could file a duplicate of already-open work. Use gh search issues --state all (spans issues and PRs) or add a separate gh pr list search.

strata-documentation-engine/skills/create-issue/SKILL.md

Lines 28 to 31 in 4e83b4e

	GitHub numbers issues and PRs together, so `--state all` surfaces PRs too; a PR may already be doing
	the work.

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[jeffhorn-nava]

Fixed in 66cf00b. gh issue list never returns PRs regardless of --state, so the step now uses gh search issues --include-prs, with --state omitted to cover both open and closed. I also corrected the definition of done in #19, which had prescribed the --state all command this bug originated from.