Discover extensions through Python entry points

[codeforester]

Parent: #50

Train position: 14 of 22
Phase: P2 — Operational completeness

Problem

There is no standard package-level plugin discovery mechanism for commands, profiles, or framework extensions.

Acceptance criteria

• Documented Python entry-point groups cover command, profile, and plugin extension types.
• Discovery is lazy, deterministic, cacheable, and has explicit collision, version, ordering, and metadata rules.
• Broken plugins produce actionable isolated errors; discovery can be disabled or allowlisted.
• Tests install a real sample distribution and verify nested commands, cache behavior, collisions, and failures.

Dependencies

• Attach the lifecycle to existing nested and lazy Click command trees #57
• Publish stable typed runtime, profile, and context extension contracts #59