feat: Add agent-friendly CLI output and discovery - #6903

Summary

• Add a proposed RFC for an agent-friendly Feast CLI covering the complete P0–P3 roadmap without external-product comparison; implement P0/P1 now, with P2/P3 explicitly deferred.
• Add opt-in root --output json|yaml with versioned success/error envelopes, stable error codes, stdout isolation, and explicit rejection for unsupported commands. Add feast commands for parameter/capability discovery and structured list/describe for pilot read commands.
• Add structured local SQLite plan/apply reporting, fix reversed feature JSON names, and ensure invalid inputs and provider-login failures do not report success. Preserve legacy default output and command-local --output semantics.
• Update CLI reference/navigation and add focused plus local subprocess tests.

Validation

• 69 focused/regression tests passed.
• 4 self-contained local subprocess integration tests passed (with --noconftest because the local environment lacks testcontainers).
• Ruff lint and format checks, git diff --check, and scoped mypy checks passed (import-untyped disabled for missing local third-party stubs).
• Full legacy CLI suite was not run because the local environment lacks assertpy.

Scope and limitations

• Structured plan/apply currently supports the local provider with SQLite online storage and automatic baselines disabled; other configurations fail before importing feature-repository code.
• Existing unrelated untracked local files were not included.