feat: elevate a hotfix commit set across the env chain in the generated workflow

[joshua-temple]

Problem

The generated hotfix workflow applied one commit to one environment per dispatch. PR #246 taught the planner to handle a commit set across the bottom-up environment chain; this PR wires the generated cascade-hotfix.yaml to drive it.

Fix

• The commit dispatch input now accepts a comma-delimited set of refs or short/long SHAs.
• The plan job calls cascade hotfix plan --commits ... --target-env ... so the Go planner is the sole source of truth for env expansion and per-(commit, environment) idempotency; the apply job loops the planner's env_sequence bottom-up and cherry-picks each environment's computed commit list. Shell iterates, it re-implements no planner logic.
• On a cherry-pick conflict at an intermediate environment the chain HALTS and opens the existing labeled resolution PR whose body names the environments still pending and instructs re-engaging the workflow targeting the original environment. Best-effort across environments.
• No dispatch toggle needed: a single-commit, single-environment dispatch yields a one-entry sequence and is behaviorally identical to the prior single-env path.

Verification

go build/test green (1672 passed); golangci-lint clean; e2e build/vet clean. Docker e2e: all 14 hotfix scenarios pass, including three new ones (multi-env clean, idempotent, conflict-halt) and the existing prod-gate and stacked scenarios (no regression). schema_version unchanged.

Closes #245.