feat: add Claude Opus 4.6 model to MAX_TOKENS registry (#2205)

Add claude-opus-4-6 (released 2026-01-20) across all provider
prefixes: anthropic/, vertex_ai/, bedrock/, and bare model names.

Author: hholst80

pr_agent/algo/__init__.py

@@ -71,6 +71,7 @@
     'vertex_ai/claude-opus-4@20250514': 200000,
     'vertex_ai/claude-opus-4-1@20250805': 200000,
     'vertex_ai/claude-opus-4-5@20251101': 200000,
+    'vertex_ai/claude-opus-4-6@20260120': 200000,
     'vertex_ai/claude-opus-4-6': 200000,
     'vertex_ai/claude-3-5-sonnet@20240620': 100000,
     'vertex_ai/claude-3-5-sonnet-v2@20241022': 100000,
@@ -108,6 +109,7 @@
     'anthropic/claude-opus-4-1-20250805': 200000,
     'anthropic/claude-opus-4-5-20251101': 200000,
     'anthropic/claude-opus-4-6': 200000,
+    'anthropic/claude-opus-4-6-20260120': 200000,
     'anthropic/claude-3-5-sonnet-20240620': 100000,
     'anthropic/claude-3-5-sonnet-20241022': 100000,
     'anthropic/claude-3-7-sonnet-20250219': 200000,
@@ -116,6 +118,7 @@
     'claude-opus-4-1-20250805': 200000,
     'claude-opus-4-5-20251101': 200000,
     'claude-opus-4-6': 200000,
+    'claude-opus-4-6-20260120': 200000,
     'claude-3-7-sonnet-20250219': 200000,
     'anthropic/claude-3-5-haiku-20241022': 100000,
     'anthropic/claude-haiku-4-5-20251001': 200000,
@@ -126,6 +129,7 @@
     'bedrock/anthropic.claude-3-sonnet-20240229-v1:0': 100000,
     'bedrock/anthropic.claude-opus-4-20250514-v1:0': 200000,
     'bedrock/anthropic.claude-opus-4-1-20250805-v1:0': 200000,
+    'bedrock/anthropic.claude-opus-4-6-20260120-v1:0': 200000,
     'bedrock/anthropic.claude-opus-4-6-v1:0': 200000,
     'bedrock/anthropic.claude-3-haiku-20240307-v1:0': 100000,
     'bedrock/anthropic.claude-3-5-haiku-20241022-v1:0': 100000,
@@ -137,6 +141,7 @@
     'bedrock/anthropic.claude-sonnet-4-5-20250929-v1:0': 200000,
     "bedrock/us.anthropic.claude-opus-4-20250514-v1:0": 200000,
     "bedrock/us.anthropic.claude-opus-4-1-20250805-v1:0": 200000,
+    "bedrock/us.anthropic.claude-opus-4-6-20260120-v1:0": 200000,
     "bedrock/global.anthropic.claude-opus-4-5-20251101-v1:0": 200000,
     "bedrock/us.anthropic.claude-opus-4-5-20251101-v1:0": 200000,
     "bedrock/global.anthropic.claude-opus-4-6-v1:0": 200000,