[codex] Add on-demand high-fidelity skills

[mimeding]

Summary

• Add backward-compatible Osaurus skill activation metadata for discoverable, default-selected, and on-demand behavior.
• Persist skills loaded through capabilities_load in session state so they stay available for the chat without becoming globally selected.
• Keep high-fidelity skills out of startup prompts unless selected or loaded, while indexing them for capabilities_search.
• Add a portable first-party high-fidelity skill pack under skills/first-party with .claude-plugin/marketplace.json import metadata.
• Update public docs to match the post-Deprecate Work Mode (migrate into single Chat / Agent system) #893 single Chat/Agent loop and lightweight activation model.

Validation

• swift build from Packages/OsaurusCore with the repo rule filter: no non-IkigaJSON Swift errors.
• swift build --package-path Packages/OsaurusCLI: passed.
• swift build --package-path Packages/OsaurusRepository: passed.
• swift test --filter "SkillActivationMetadataTests|FirstPartySkillPackTests|SessionPreflightCacheTests|CapabilityLoadBufferTests|CapabilitiesLoadToolTests|SkillSearchServiceTests": passed, 31 tests.
• swift test --package-path Packages/OsaurusCLI --parallel: passed, 32 tests.
• jq empty .claude-plugin/marketplace.json: passed.
• git diff --check: passed.
• find scripts -name '*.sh' -print0 | xargs -0 shellcheck --severity=warning: passed.
• swiftlint lint --reporter github-actions-logging: exit 0 with existing repo warnings.

Notes

• Assumes PR Deprecate Work Mode (migrate into single Chat / Agent system) #893 is implemented: this targets the single Chat/Agent loop, not legacy separate-mode flows.
• Direct push to osaurus-ai/osaurus was denied for the authenticated account, so this draft PR is opened from the mimeding/osaurus fork.

[mimeding]

CI note: test-core failed in the Xcode build phase before running tests with EventSource module-resolution errors: CAsyncHTTPClient, CNIOLLHTTP, CNIOExtrasZlib, CNIOPosix, and _NumericsShims. This matches the known EventSource/toolchain failure class from the recovery plan, not a failure in the new skill tests. Local package builds were clean, and the focused skills/session/capabilities tests passed. Leaving this PR as draft until test-core is resolved or rerun under a fixed CI/toolchain baseline.

[mimeding]

Recovery-plan note: the latest CI shape is swiftlint, shellcheck, and test-cli passing, with test-core failing in the shared EventSource module-resolution class. #975 is green and should be merged first, then this PR should be rerun before touching the high-fidelity skills code.

This PR also remains aligned with the lightweight activation strategy: installed/discoverable by default, no high-fidelity skill prompt injection unless selected or loaded on demand.