refactor: use base_model syntax for gitlab fable 5

Inherit metadata from anthropic/claude-fable-5 instead of duplicating
it, matching the amazon-bedrock fable definition. Override only the
gitlab-specific name, the required reasoning_options, and cost = 0
(proxy-billed).

providers/gitlab/models/duo-chat-fable-5.toml

@@ -1,26 +1,9 @@
+base_model = "anthropic/claude-fable-5"
 name = "Agentic Chat (Claude Fable 5)"
-description = "Claude Fable model for coding, reasoning, and agentic workflows"
-family = "claude-fable"
-release_date = "2026-06-09"
-last_updated = "2026-06-09"
-attachment = true
-reasoning = true
-reasoning_options = []
-temperature = false
-tool_call = true
-knowledge = "2026-01-31"
-open_weights = false
+reasoning_options = [{ type = "effort", values = ["low", "medium", "high", "xhigh", "max"] }]
 
 [cost]
 input = 0
 output = 0
 cache_read = 0
 cache_write = 0
-
-[limit]
-context = 1_000_000
-output = 128_000
-
-[modalities]
-input = ["text", "image", "pdf"]
-output = ["text"]