Add BLIGHT:: document-scope triggers and case-insensitive matching

- Split TRIGGER_PATTERN into INLINE_PATTERN (BLIGHT:) and DOCUMENT_PATTERN
  (BLIGHT::), both case-insensitive
- Inline triggers replace only the trigger line (existing behaviour)
- Document-scope triggers replace the entire file; multiple BLIGHT:: triggers
  in one file are processed sequentially, each seeing the previous result
- Updated FAILED_TEMPLATE to two-line format with BLIGHT_FAILED and BLIGHT_ERROR
- Added complete_document() to AIProvider ABC and GeminiProvider with a
  dedicated system prompt instructing the model to return the full document

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ai/gemini.py

@@ -2,7 +2,7 @@ import google.generativeai as genai
 import config
 from .base import AIProvider
 
-_SYSTEM_PROMPT = (
+_INLINE_SYSTEM_PROMPT = (
     "You are an inline document assistant. "
     "The user will provide a markdown document and a specific instruction. "
     "Your response must contain ONLY the text to be inserted into the document — "
@@ -11,13 +11,26 @@ _SYSTEM_PROMPT = (
     "Respond as if your output will be dropped directly into the middle of a document."
 )
 
+_DOCUMENT_SYSTEM_PROMPT = (
+    "You are a document editing assistant. "
+    "The user will provide a markdown document and a specific instruction. "
+    "Apply the instruction to the entire document and return the full rewritten document. "
+    "Your response must contain ONLY the rewritten document — "
+    "no preamble, no explanation, no meta-commentary, no markdown code fences. "
+    "Preserve the document's structure and formatting unless the instruction says otherwise."
+)
+
 
 class GeminiProvider(AIProvider):
     def __init__(self) -> None:
         genai.configure(api_key=config.GEMINI_API_KEY)
-        self._model = genai.GenerativeModel(
+        self._inline_model = genai.GenerativeModel(
             model_name="gemini-2.5-flash-lite",
-            system_instruction=_SYSTEM_PROMPT,
+            system_instruction=_INLINE_SYSTEM_PROMPT,
+        )
+        self._document_model = genai.GenerativeModel(
+            model_name="gemini-2.5-flash-lite",
+            system_instruction=_DOCUMENT_SYSTEM_PROMPT,
         )
 
     def complete(self, document: str, instruction: str) -> str:
@@ -25,5 +38,13 @@ class GeminiProvider(AIProvider):
             f"DOCUMENT:\n\n{document}\n\n"
             f"INSTRUCTION: {instruction}"
         )
-        response = self._model.generate_content(prompt)
+        response = self._inline_model.generate_content(prompt)
+        return response.text.strip()
+
+    def complete_document(self, document: str, instruction: str) -> str:
+        prompt = (
+            f"DOCUMENT:\n\n{document}\n\n"
+            f"INSTRUCTION: {instruction}"
+        )
+        response = self._document_model.generate_content(prompt)
         return response.text.strip()