Remove epic.sc; move its docs pass into implement-enhanced.sc

[adamw]

epic.sc was implement.sc with three agents pinned to concrete accessors and a trailing docs stage:

Step	implement.sc	epic.sc
Plan	planningAgent	claude.opus
Session	codingAgent	claude
Reviewers	reviewAgent	codex
Per-task loop	identical	identical
Docs pass	—	stage("Update documentation")

The first three differences are pure configuration — this settings file makes implement.sc behave identically:

planningAgent = claude:opus
codingAgent   = claude
reviewAgent   = codex

So the flow demonstrated a capability rather than providing one, at the cost of a fourth near-identical plan-then-implement entry in a picker where the one-line description is all the user sees. Its name didn't help either: every flow here is multi-task, and epicId already means something else in Plan.

The docs stage was the only thing it did that no other flow does. It moves to implement-enhanced.sc (the full-service variant), placed before openPrFromBranch so the docs land in the PR rather than in a separate commit — and as its own stage, keeping the push a later stage than the edits it pushes (ADR 0018 §3.2 R8). Its description line now reads // Plan (self-reviewed), implement per task, update docs, then open a PR.

Follow-on reference updates: flow lists in README.md and examples/runnable/README.md; the expected index in BuiltInFlowsTest; flowScripts in ScalaCliSmokeTest; and FlowCompilesTest.epicFlowShape renamed to crossBackendReviewShape (the API surface it compile-checks is still worth keeping). README.md's concrete-accessor example now cites issue-pr-bugfix.sc, which pins claude.sonnet for its reproduction checks. ADR references are left alone as historical record.

Verified: scala-cli compile flows/implement-enhanced.sc, sbt scalafmtCheckAll, the shell flow suites, and runner/Test/compile all pass.