hotfix a single component without touching its siblings

[joshua-temple]

Problem / why

Hotfix is environment-major and uses a global tag and branch namespace today
(internal/hotfix/finalize.go, internal/hotfix/lifecycle.go). For a multi-component manifest,
a hotfix must diverge only one component's environment while siblings keep tracking trunk.

Scope (firm intent, to be refined after P1)

• Key hotfix divergence (Ref / BaseSHA / Patches) under the component dimension.
• Segment the branch namespace per component: EnvBranchPrefix becomes env/<comp>/<env>.
• Scope hotfix tag queries (HotfixTagsForBase) to the component's tag_prefix namespace,
  replacing the global tag enumeration.
• Port the hotfix finalize overlay to state.components[comp][env].

Files / areas touched

• internal/hotfix/finalize.go, internal/hotfix/lifecycle.go,
  internal/promote/finalize.go (rejoin block).

Tests and coverage required

• e2e: a hotfix diverges only A's environment while B tracks trunk.
• A single-component regression test that hotfix is unchanged.

Acceptance criteria

• A per-component hotfix affects only that component's branch / tag namespace and state subtree.
• Single-component hotfix is unchanged.

Dependencies

Blocked by the independent promotion milestone.