fix(scaffolde): migrate Gmail capability identity

[pai-scaffolde]

Summary

• advertise the canonical scaffolde.gmail capability identity
• retire and reject scaffolde.gmail.pai descriptors
• remove account-specific PAI wording from consumer prompts and schema examples
• verify old-ID invocation returns unknown_capability against a valid new registry

Verification

• scripts/run_tests.sh tests/tools/test_scaffolde_capability_tool.py -q — 17 passed
• Ruff — passed
• Ty — passed
• independent exact-commit review — SHIP
• repository-wide run: 13,316 passed; 72 unrelated optional-integration tests failed because the local dev-only environment lacked their platform extras; no changed test failed

Landing order

Consumer-first prerequisite for the coordinated Scaffolde producer migration.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep legacy descriptors valid during the consumer-first rollout

When an existing installation still has the previously supported scaffolde.gmail.pai descriptor, this new validation error makes the registry degraded; invoke_capability() then returns capability_degraded before looking up any operation, and the capability prompt is omitted. Because this commit is intended to land before the coordinated producer migration, upgrading Hermes immediately disables Gmail for every installation whose producer has not yet rewritten the registry. Accept or translate the legacy ID during the transition, and retire it only after producers have migrated.

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