feat(openai): add GPT-5.5 and GPT-5.5 Pro (#70)

Source: https://openai.com/index/introducing-gpt-5-5/ (announcement 2026-04-23)

Adds 4 new model entries across 3 provider files:

- providers/openai.toml:
    gpt-5.5      — 1M context,  $5/$30 per 1M tokens (input/output)
    gpt-5.5-pro  — 1M context, $30/$180 per 1M tokens
- providers/codex-cli.toml:
    codex-cli/gpt-5.5  — 400K context (Codex subscription limit),
                         $0/$0 (covered by subscription)
- providers/chatgpt.toml:
    gpt-5.5-codex  — 400K context, session-auth (subscription)

Notes:
- API availability announced as 'very soon'; pricing confirmed in the
  announcement. tools/vision/streaming flags match GPT-5.4 family.
- max_output_tokens retained at 128000 (openai/codex-cli) and 65536
  (chatgpt) — matches sibling 5.4 entries; announcement does not
  specify a new output cap.
- Header comment in openai.toml updated (15 → 17 models).

providers/chatgpt.toml

@@ -1,5 +1,5 @@
 # ChatGPT (Session Auth / Codex Responses API) — https://chatgpt.com
-# Models: 5 (free with ChatGPT subscription via OAuth session)
+# Models: 6 (free with ChatGPT subscription via OAuth session)
 
 [provider]
 id = "chatgpt"
@@ -8,6 +8,21 @@ api_key_env = "CHATGPT_SESSION_TOKEN"
 base_url = "https://chatgpt.com/backend-api"
 key_required = true
 
+[[models]]
+id = "gpt-5.5-codex"
+display_name = "GPT-5.5 Codex"
+tier = "frontier"
+# Per OpenAI's GPT-5.5 announcement, Codex exposes a 400K context window
+# for subscription plans (announcement 2026-04-23).
+context_window = 400000
+max_output_tokens = 65536
+input_cost_per_m = 0.0
+output_cost_per_m = 0.0
+supports_tools = true
+supports_vision = false
+supports_streaming = true
+aliases = ["chatgpt-5.5"]
+
 [[models]]
 id = "gpt-5.4-codex"
 display_name = "GPT-5.4 Codex"