docs(spec): unify SPEC at v0.6 (two profiles) + parser accepts 0.6

[Chosen9115]

Summary

• Folds the v0.2 additive diff into one authoritative SPEC.md at v0.6, documenting both execution profiles (Rails runtime .sop.yaml + CLI local .sop.json) in a single capability matrix.
• Parser SUPPORTED_SPEC_VERSIONS → [0.1 0.2 0.6]; process files declare opensop: "0.6".
• Documents the local backend (cells, pause/resume, run artifacts) and executor: internal|external (audit metadata only). Stubbed server features (judgment/subprocess/notification/wait/webhook-poll) marked honestly.

Self-rating (per project workflow)

• correctness 9 · simplicity 8 · test coverage 8 · naming 9 · performance risk 9 · security risk 9 · process-model fidelity 9
• Lowest = test coverage/simplicity: the spec is large; the parser change is minimal and directly tested. Improvement applied: added parser spec cases asserting 0.6 parses and unknown versions still error.
• Deferred follow-up: the SPEC documents local step-type parity that the CLI implements (v0.7.0, feat(local): v0.7.0 — full local step types + pause/resume opensop-cli#15); a couple of low webhook parity gaps tracked separately.

Test plan

• bundle exec rspec spec/services/opensop/definition_parser_spec.rb → 74 examples, 0 failures.

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