[codex] Document high-fidelity business file workflows

[mimeding]

Business rationale

Clarifies how high-fidelity business-file workflows should activate only when relevant, and gives users explicit PPTX capability wording for native versus enhanced flows.

Coding rationale

Keeps the slice docs-only. Documents the current internal document adapter/emitter split without claiming unavailable plugin C ABI callbacks, and captures optional LibreOffice/OpenOffice detection as enhancement guidance rather than a hard dependency.

What changed

• Added high-fidelity business-file skill activation guidance for presentation-content-producer and document-data-analyst.
• Added plugin-authoring guidance for the intended parser/emitter split and a concise PPTX plugin example.
• Added docs/OFFICE_RUNTIME.md for optional office runtime detection, enhanced validation/export/preview flows, and install hint wording.
• Updated docs/FEATURES.md with the Business File Runtime inventory entry, PPTX capability model, and documentation index link.

Validation

• git fetch origin && git rebase origin/main
• Checked repo for Markdown/doc lint commands; no Markdown lint target found.
• git diff --check
• Link/path sanity pass with rg and test -e for referenced local docs/code paths.

Non-scope

• No Swift or plugin ABI code changes.
• No LibreOffice/OpenOffice bundling, installer, or runtime implementation.
• No changes to native PPTX generation behavior.

Residual risks

• The future external plugin parser/emitter ABI may use names or shapes different from the intended split documented here.
• Enhanced office-runtime behavior remains documentation guidance until implementation lands.

[mimeding]

CI note: current test-core red is the shared ModelPickerItemCache / RemoteProviderManagerRefreshTests singleton race, not this branch's implementation. This is a docs-only branch; the failing tests are provider/cache tests. The race is isolated and fixed in #1025; after that lands, this branch should just need a rebase/rerun.

[mimeding]

@tpae PR-specific CI note / next step:

This is a docs-only PR. Its test-core failure is the shared provider/model-picker race currently present on main, not a docs regression.

Required next step: merge the green base fix in #1025 first, then I will rebase/rerun this PR so the checks can reflect this branch's actual diff.

Why: docs-only changes cannot cause the failing RemoteProviderManagerRefreshTests / ModelPickerItemCacheTests; rerunning before #1025 lands just repeats the known red-base failure.