generate per-component promotion workflows and concurrency groups

[joshua-temple]

Problem / why

Promotion workflows are generated globally (internal/promote/promote.go emission). For a
multi-component manifest, like orchestrate (V3), promotion must fan out to one file per
component so each component's promotion runs and concurrency are isolated. A single-component
manifest keeps promote.yaml.

Scope (firm intent, to be refined after P1)

• Emit one promote workflow file per component (promote-<name>.yaml; the single-component
  default keeps promote.yaml).
• Per-component concurrency group, replicating the V3 / external pattern.
• Reserve the matrix for legs WITHIN a component (the existing per-environment / per-promotion
  matrix), not across components.

Files / areas touched

• internal/promote/promote.go (emission), internal/generate/command.go (fan-out).

Tests and coverage required

• Generator unit tests: one promote file per component, each with its own concurrency group.
• actionlint-clean on generated multi-component promote workflows.
• A single-component regression test that promote.yaml is unchanged.

Acceptance criteria

• One promote workflow per component, each with a distinct concurrency group, actionlint clean.
• Single-component promote output is unchanged.

Dependencies

Blocked by P1.

[joshua-temple]

Implemented and merged via #518.