SIF-003: package and diagnose third-party stage plugins

[jmrpineda]

Reported by: codex
Requested by: jmr.pineda
Priority: P2
Affected surfaces: plugin packaging, plugin diagnostics, third-party extensibility
Constraints: preserve the versioned plugin contract and avoid making external plugins second-class citizens

Summary

Add external plugin packaging and diagnostics so third-party stage plugins can be distributed, loaded, and debugged with a supported operational path.

Problem / opportunity

The runtime already exposes a plugin contract, but external plugin adoption stays fragile without a clear packaging model and diagnostics for discovery, compatibility, and failure analysis.

Requested behavior

Third-party plugin authors should have a documented and supported way to package plugins, and operators should receive useful diagnostics when plugin loading or capability negotiation fails.

Scope

• In scope: packaging format or conventions, compatibility diagnostics, load-time validation, operator-facing error clarity.
• Out of scope: a public marketplace or a full hosted plugin ecosystem.

Acceptance criteria

1. Third-party plugins can be packaged using a documented supported convention.
2. Plugin loading failures surface actionable diagnostics instead of opaque errors.
3. Capability or contract mismatches are detected and reported clearly.

Notes

• Source: local roadmap at .doc/positioning-and-roadmap.md.
• This issue was created while migrating local backlog ownership to GitHub.