Clarify v0.4 extension event namespaces and draft governance

[pramodbn27]

What changed

This PR tightens the v0.4 draft schema and updates the surrounding spec docs to match.

It includes:

• a dedicated extension_name definition in specification/v0.4/schemas/common.schema.json
• event_name support for either:
  • canonical aiops.* names, or
  • reverse-domain namespaced extension events
• an explicit rule that extension names must not collide with the aiops namespace
• new valid and invalid v0.4 fixtures covering extension-event behavior
• test updates so the canonical aiops.* catalog remains checked against v0.3 semantic conventions
• README / ROADMAP / CONTRIBUTING clarifications around:
  • vendor-neutrality still being an intended end state rather than a closed acceptance gate
  • draft snapshot tagging expectations
  • non-normative future work around OTel GenAI bindings

Why it changed

The schema already supported extension objects, but the event-name boundary between canonical AIOS events and third-party extension events was still under-specified.

That left two important ambiguities for implementers:

• what shape a valid extension event name should have
• whether a producer could mint custom names that collide with the reserved aiops namespace

This PR makes that boundary explicit and testable.

Impact

Independent producers and consumers now have a clearer rule:

• canonical reviewed names stay under aiops.*
• extension events must use reverse-domain namespacing and must not begin with aiops

That improves interoperability without claiming that v0.4 is frozen or stable.

Commit structure

1. Define extension event namespaces in v0.4 schema
2. Clarify spec governance and draft tagging

Validation

Ran from the branch tip:

./.venv/bin/pytest

Results:

• pytest passed (20 tests)

Notes

• This PR is intentionally framed as a draft-spec clarification, not a package release bump.
• pyproject.toml remains at 0.1.0, which is consistent with the repository still being a pre-release draft specification rather than a frozen milestone release.
• Opened as a draft PR to make it easy to review the schema rule and fixture changes before finalizing wording.

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