feat(protocol): negotiate versions during initialization

[manavanandani]

Summary

• introduce independent MAJOR.MINOR protocol version negotiation in agent/initialize
• preserve versionless agents as legacy protocol 0.1 with one warning per run
• abort before step execution on incompatible major versions with VERSION_UNSUPPORTED (-32001)
• update conformance tooling, the pytest fixture, Inspector initialization, audit metadata, schemas, and documentation

Compatibility

• differing minor versions negotiate to the lower supported minor
• agents without protocol_version continue to run under the legacy 0.1 assumption
• the current SDK remains compatible with pre-negotiation runtimes
• malformed or explicitly null versions are rejected as invalid parameters (-32602)

Validation

• python -m unittest discover -s runtime/python/tests — 84 tests
• python -m unittest discover -s sdk/python/tests — 42 tests
• ruff check runtime/python/src runtime/python/tests sdk/python/src sdk/python/tests examples/protocol_demo.py
• mkdocs build --strict
• node --check server/src/server.js
• Draft 2020-12 validation for all JSON Schemas and current/legacy initialize payloads
• runtime and SDK source-distribution and wheel builds
• real stdio interoperability checks for legacy agents, minor negotiation, and major-version aborts

Closes #25