feat: add allow_breaking_changes manifest field to disable the breaking-change gate

[joshua-temple]

Problem

The breaking-change promote gate (feat!: / BREAKING CHANGE: commits block the prerelease-to-release and release-to-prod boundaries) can only be bypassed per-run today, via the --allow-breaking flag or the allow_breaking_changes workflow input. A team that never wants the gate has no way to turn it off once. Closes #492.

Fix

Add an additive, optional manifest field allow_breaking_changes (bool, default false = gate enabled, so existing behavior is byte-identical). When true, the gate is skipped for the repo without the per-run override; the per-run --allow-breaking flag and workflow input keep working as before.

The gate is enforced in two places, both handled:

• Go preflight (checkBreakingChangesForMode): short-circuits on the manifest field. The generated promote.yaml delegates to cascade promote preflight, so the CLI honoring the manifest covers the promote path.
• Generated release shell (release.go): independently gates in bash off ALLOW_BREAKING. The value is now baked from the manifest at generation time (ALLOW_BREAKING: "true" when the field is set), leaving the operator input declared. Default output is byte-identical (no drift in cascade's own workflows).

Verification

• go build ./..., go test ./... -race, golangci-lint run ./... all clean.
• JSON schema declares the field across all three byte-identical copies (internal/schema, schema/, docs/public/), guarded by TestSchema_OnDiskCopiesAreByteIdentical.
• New e2e scenario 49-allow-breaking-changes-manifest drives a feat!: commit through promote with the manifest field set and no per-run override.
• Docs: manifest reference + capability description + coverage-matrix row updated in the same PR.

Closes #492.