feat(spec): remove extends support

[rohangpta]

Summary

• removes SPEC inheritance from the compiler and compiled IR
• removes extends-only local workspace artifact and session-manifest machinery
• stops registry skill discovery from traversing parent specs
• removes the unused Harbor adapter option
• rejects an extends field explicitly instead of silently ignoring it

This belongs in the telos repository: Cloud has no independent extends implementation and consumes the Telos runtime contract.

Compatibility

Existing session JSON remains readable because retired fields are ignored during decoding. New plans, applies, and runs containing extends fail with a direct unsupported-field error.

Merge after #35 so the public telos-cli documentation lands before the implementation is retired.

Validation

• go vet ./...
• go test ./...
• go test -race ./...
• python3 -m unittest integrations.harbor.telos_agent_test
• ruff check integrations/harbor/telos_agent.py integrations/harbor/telos_agent_test.py
• ruff format --check integrations/harbor/telos_agent.py
• git diff --check