feat(pi): add Codex fast mode extension

pi/extensions/openai-codex-fast/README.md

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+# OpenAI Codex fast mode
+
+Adds `/fast` for Pi's `openai-codex` provider and `openai-codex-responses` API.
+When enabled, the extension adds `service_tier: "priority"` to matching requests.
+It keeps the selected model, reasoning level, tools, and conversation input.
+
+## Usage
+
+- `/fast` toggles fast mode.
+- `/fast on` requests priority service. Usage may increase.
+- `/fast off` stops this extension's override and restores provider defaults.
+- `/fast status` reports the setting without changing it.
+
+New sessions start with fast mode off. Changes are stored as custom session
+entries, outside model context. Resume and reload restore the active branch's
+setting. Forks inherit only settings before the fork point. Switching to another
+provider makes the setting inactive; switching back to Codex reactivates it.
+Changes require an idle session. Status reports remain available during a turn.
+
+The footer shows `Codex fast: priority` while this extension requests priority
+service. This shows the requested tier, not confirmation from OpenAI.
+
+## Availability and usage
+
+OpenAI controls priority access for each model and account. Priority service can
+increase subscription usage or cost. The extension does not change reasoning
+settings, query Codex's model catalog, verify account eligibility, or guarantee
+faster responses. Unlike the Codex CLI, it does not hide the command for models
+without an advertised fast tier. API failures use Pi's normal error handling;
+the extension does not silently retry with another tier.
+
+Other providers are untouched, including `openai`, OpenRouter, and providers
+that reuse the Codex API under another name. While enabled, this extension
+replaces any existing `service_tier` value with `priority`. Disabling it leaves
+native request settings and other extensions' overrides unchanged.
+
+Your `openai-server-compaction` extension can remain enabled. Both request
+patches preserve unrelated fields. Fast mode applies to normal session requests,
+not the compaction extension's separate HTTP compaction or summary calls.
+
+## Installation
+
+Home Manager deploys this directory through `rsydn.aiTools.piExtensions` along
+with the other extensions. Apply your usual dotfiles rebuild, then run `/reload`.
+To test before deploying:
+
+```sh
+pi -e ./pi/extensions/openai-codex-fast/index.ts
+```
+
+## Codex implementation reference
+
+Reviewed OpenAI Codex commit
+[`1a69ad3237fc`](https://github.com/openai/codex/tree/1a69ad3237fc485273ccfa9731239fdf789b43e4).
+
+- [Service-tier UI](https://github.com/openai/codex/blob/1a69ad3237fc485273ccfa9731239fdf789b43e4/codex-rs/tui/src/chatwidget/service_tiers.rs)
+  toggles the model's advertised Fast tier and persists the selection.
+- [Model catalog](https://github.com/openai/codex/blob/1a69ad3237fc485273ccfa9731239fdf789b43e4/codex-rs/models-manager/models.json)
+  identifies Fast with the request value `priority`.
+- [Request builder](https://github.com/openai/codex/blob/1a69ad3237fc485273ccfa9731239fdf789b43e4/codex-rs/core/src/client.rs)
+  sends the selected tier in `service_tier`.
+
+Pi's `before_provider_request` hook changes the native payload. No provider
+replacement, custom transport, or additional runtime dependency is required.
+
+## Tests
+
+Requires Node.js 22.19 or newer and an installed `pi` executable. Tested with
+Pi 1.1.0.
+
+```sh
+node --experimental-strip-types --test \
+  pi/extensions/openai-codex-fast/tests/*.test.ts \
+  pi/extensions/openai-codex-fast/tests/*.test.cjs
+```
+
+Unit tests cover scope, payload preservation, branch state, and both compaction
+patch orders. The runtime test uses Pi's real loader and native providers with a
+local HTTP fixture. It checks outgoing payloads, commands, the idle guard,
+provider switching, resume, and forks. It sends no requests to OpenAI and uses no
+real credentials.