feat: reserve canary and blue-green deploy fields on rollout

[joshua-temple]

Problem

Issue #1 asked to reserve canary and blue-green deploy controls in the manifest. An earlier draft added a separate strategy: block with sibling canary:/blue_green: sub-blocks on each deploy. That collided with the repo's already-canonical reservation: RolloutConfig (rollout:) at internal/config/schema_v1.go already models type: rolling|canary|blue_green with reserved canary and blue_green sub-blocks, and the schema comment states there is no shared strategy: block. Two surfaces for the same concept would have been redundant and contradicted that comment.

Fix

Converge onto the canonical rollout surface. The useful fields from #1 are reserved by enriching the existing rollout.canary block, additively and omitempty:

• percent (int, 1..100) single initial canary weight
• bake_time (Go duration string) soak before promotion
• promote_callback / rollback_callback local callback workflow paths

rollout.blue_green keeps its existing switch field as the cutover workflow path (issue #1's cutover_callback is the same concept; no duplicate field is added). No strategy: surface is introduced.

These are reserved shape only: the generator does not read them, so generated workflow bytes are identical whether the fields are present or absent. Because the change is additive, schema_version is unchanged.

Verification

• go build ./..., go test ./... (1475 pass), golangci-lint run ./... (no issues) all green.
• e2e module builds and vets clean; new scenario e2e/scenarios/23-canary-bluegreen-reserved.yaml exercises a canary and a blue-green rollout and proves regeneration produces no drift (verify exit 0).
• Validation tests cover percent range, bake_time duration parsing, and callback-path safety via the existing helper. A generator test asserts byte-identical output with vs without the new fields populated. The three JSON schema copies are regenerated in sync.
• Docs: versioning.md documents the enriched rollout reserved shape and reaffirms there is no strategy: block.

Closes #1