[feat] Add /capture command and product-manager persona for portable issue capture

[lugassawan]

Summary

• Adds agents/product-manager.md: persona-led subagent that turns a rough thought into a well-framed GitHub issue; applies four PM lenses (problem, value, acceptance criteria, Impact/Effort S/M/L); discovers issue templates at runtime via ls .github/ISSUE_TEMPLATE/; falls back to a default body shape when none exist; sanitizes dup-scan keywords and temp-file slug against shell injection; binds the confirm step to a .cmd sidecar file so the filed command matches exactly what the user reviewed
• Adds commands/capture.md: thin orchestrator that echoes $ARGUMENTS, delegates to product-manager, enforces the preview-gate contract (no gh issue create until user replies confirm), and carries no ticket-context prelude or plan-output footer
• Both files are repo-agnostic: no hardcoded template names, no hardcoded repo owner, no --label/--assignee/--milestone/--repo flags; works in any repo where the swe-workbench plugin is installed

Test Plan

• Changed skills/commands/agents load without errors
• Examples in the diff were exercised manually
• §1 — /swe-workbench:capture appears in fresh Claude Code session; product-manager agent resolves
• §2 — Preview shows Filing into: lugassawan/swe-workbench exactly
• §3 — Happy path (feature): add a /retro command → feature_request.md selected, ## Product framing prepended, four lenses populated; reply confirm → issue URL returned
• §4 — Happy path (bug): cleanup-merged sometimes leaves stale local branches → bug_report.md, problem framing emphasizes broken-state symptom
• §5 — Happy path (chore): bump dependabot interval to weekly → chore_maintenance.md, value framing acknowledges internal cleanup
• §6 — Ambiguity gate: the workflow feels slow → agent asks ONE clarifying question before drafting
• §7 — Preview gate: run any happy path, do NOT reply confirm; verify NO issue created (gh issue list --limit 5)
• §8 — Dup detection: pre-create stub issue with unique keyword; /capture with that keyword surfaces it in "possibly related"
• §9 — Auth bail: GH_TOKEN=invalid /capture … → clean single-line bail before any framing
• §10 — Framing-block round-trip: open issue from §3 in browser; ## Product framing appears at top with all four lenses
• §11 — No-template repo: scratch repo with no .github/ISSUE_TEMPLATE/; preview surfaces "No issue templates found"; default body used; issue files into scratch repo, NOT swe-workbench
• §12 — Custom-template repo: scratch repo with epic.md template; agent picks epic.md, not any swe-workbench-style name

Issue: N/A — captured during /design session 2026-05-02; this PR is the tracking artifact