let a component target a subset of the environment ladder

[joshua-temple]

Problem / why

Some components ship to only part of the repo's environment ladder. A shared global ladder
with a per-component subset (rather than fully independent ladders) keeps the promotion walk a
simple filtered list walk.

Scope (firm intent)

• Activate ComponentConfig.Environments as an optional subset of the global ladder.
• Add per-component env helpers (ComponentEnvironments(name) and per-component variants of
  GetNextEnvironment / GetEnvironmentsInRange).
• The promotion walk filters the global ladder to the component's subset.

Files / areas touched

• internal/config/types.go (subset resolver and per-component env helpers),
  internal/promote/promote.go (filtered walk).

Tests and coverage required

• Unit and e2e: a component with a 2-environment subset promotes only through those
  environments.

Acceptance criteria

• A component can target a subset of the global ladder and promotes only through it.

Dependencies

Blocked by the monorepo: independent promotion milestone.

[joshua-temple]

Implemented and merged: the promotion runtime now narrows to a component's resolved environment ladder (rollback and generation already did), so a component with a shorter environments list is promoted only through its own environments. PR #530.