feat(compound-engineering): assign the most relevant existing label when creating a tracker issue

[pekral]

Summary

• Adds a shared normative contract — assign the most relevant existing label when creating a tracker issue — as a new canonical section in rules/compound-engineering/general.mdc, right after Claim a tracker issue before working on it.
• Mechanism: enumerate existing labels (gh label list --json name,description on GitHub), match by type + domain, never invent a new label and never fall back to a random/first-available one; when nothing genuinely fits, create the issue unlabelled. EPIC (owned by create-issues-from-text) stays the sole sanctioned exception. JIRA is documented as a secondary path (free-text labels, acli limits); Bugsnag is explicitly out of scope.
• Thin one-line references added to the four issue-creating skills: skills/create-issue/SKILL.md, skills/create-issues-from-text/SKILL.md, skills/blueprint/SKILL.md, skills/product-capability/SKILL.md. No agent files touched.
• Content-pin tests added: tests/Installer/CompoundEngineeringContentTest.php (section heading, gh label list, no-random-label wording, EPIC exception) and tests/Installer/SkillsContentTest.php (byte-identical reference sentence per skill).
• CHANGELOG.md — one feat line under [Unreleased].

Closes #734

Test plan

• composer build green (328 tests, 100% coverage, phpcs/phpstan/skill-check all pass)
• Existing gh label create EPIC pin in tests/Installer/AgentsTest.php (lines 279-297) untouched and still passing
• README skill count unaffected (no new skill directory added)

[pekral]

Code Review

Status: clean
Counts: Critical 0 · Moderate 0 · Minor 2 · Refactoring 0
Last updated: 2026-07-15T11:33:40Z
Issue tracker summary: posted summary to issue #734

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Findings

Consolidated review — quality / architecture / optimisation (argos) + security (athena). Both Minor items are prose-hardening suggestions on the same GitHub mechanics clause; neither blocks convergence (0 Critical · 0 Moderate).

🟡 Minor 1. Instruct passing the label name as a discrete CLI argument (shell-injection hardening)

• Location: rules/compound-engineering/general.mdc — Assign the most relevant existing label when creating a tracker issue → GitHub mechanics
• Note: A GitHub label name may contain shell metacharacters (spaces, ", backtick, ;, $()). The gh issue create ... --label "<name>" guidance should state that the agent must pass the label name as a discrete argument and never interpolate it into an sh -c command string. Reading via --json name,description is safe; the residual surface is only the create command assembly. Source: athena security review.

🟡 Minor 2. Note that label name / description are untrusted data (indirect prompt-injection hardening)

• Location: rules/compound-engineering/general.mdc — same GitHub mechanics clause
• Note: gh label list --json name,description returns description, which a repo collaborator can influence, and the prose feeds it into the agent's decision context ("match by type + domain"). Blast radius is bounded (label choice only; the action is fenced to select-existing / create-unlabelled). A one-line note that name / description values are untrusted — used only to name the label, never as instructions — closes the gap. Source: athena security review.

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Summary: 0 Critical · 0 Moderate · 2 Minor · 0 Refactoring · assignment conformance: conformant · posted summary to issue #734 · reviewer comments: none