feat(repo): flow-pack E4 — /flow-epics (epic decomposition, sub-issue linking, prp-create handoff)

[w7-mgfcode]

Sub-issue of #368 (umbrella: feat(repo): integrate flow-pack methodology as the flow: command suite). Parallel after Foundation — runs concurrently with E2 and E3 after E1 #369 closes.

Purpose

Implement the /flow-epics command: phase-ordered epic issue creation from the umbrella's decomposition, linked as sub-issues via the GitHub REST API, handing off to base_prp:prp-create per epic. End state: a user can run /flow-epics #<umbrella> and receive a set of Foundation / Parallel / Release-gate epic issues linked under the umbrella, ready for PRP authoring.

Scope

• Read the umbrella issue body to extract the decomposition (epic titles + phases)
• Create N epic issues: one Foundation, N parallel, one Release gate — each with the required phase blockquote + ## Purpose + ## Sub-tasks skeleton
• Link each created epic under the umbrella via POST /repos/{owner}/{repo}/issues/{umbrella_id}/sub_issues (gh api directly — no native gh sub-issue command, see Add gh issue support for parent issues / sub-tasks cli/cli#10298)
• Idempotent check per epic: gh issue list --search "<title>" before creating — skip if already exists
• Dry-run echo + explicit approval gate per write; ~1 s rate-delay between consecutive gh api calls
• Attach labels ⊇ umbrella label set + epic; attach milestone
• Print handoff at end: → base_prp:prp-create per epic with issue number

Out of scope

• Sub-issue decomposition within each epic — delegate to issue-to-subtasks
• PRP execution — that is base_prp:prp-execute
• Implementation of any epic's feature code
• Release-gate epic (E5) — deferred until E2–E4 are implemented

Sub-tasks

To be decomposed via issue-to-subtasks when E4 is picked up.

Durable-source split

• Tracked template (to be created): docs/flow-pack/commands/flow-epics.md
• Local install (gitignored, regenerable): .claude/commands/flow/flow-epics.md

[w7-mgfcode]

E4 delivered — /flow-epics command shipped in PR #374 (merged to dev). Tracked template: docs/flow-pack/commands/flow-epics.md. Local install regenerable via cp docs/flow-pack/commands/*.md .claude/commands/flow/.