fix: correct github-copilot model limits to match Copilot API

[Nepomuceno]

Summary

Corrects token limits for 6 models in the github-copilot provider to match the authoritative values from the Copilot API (GET https://api.githubcopilot.com/models).

This PR only fixes limits — no models are added or removed.

Changes

Model	Field	Before	After (Copilot API)	Delta
claude-haiku-4.5	context	144,000	200,000	+56,000
	input	128,000	136,000	+8,000
	output	32,000	64,000	+32,000
claude-opus-4.5	context	160,000	200,000	+40,000
	input	128,000	168,000	+40,000
claude-opus-4.6	context	144,000	200,000	+56,000
	input	128,000	168,000	+40,000
	output	64,000	32,000	-32,000
claude-sonnet-4.5	context	144,000	200,000	+56,000
	input	128,000	168,000	+40,000
claude-sonnet-4.6	input	128,000	168,000	+40,000
gpt-5.2	context	264,000	400,000	+136,000
	input	128,000	272,000	+144,000
	output	64,000	128,000	+64,000

How the correct values were verified

curl -s "https://api.githubcopilot.com/models" \
  -H "Authorization: Bearer $(gh auth token)" \
  -H "Copilot-Integration-Id: vscode-chat"

Each model in the response includes:

"limits": {
    "max_context_window_tokens": ...,   // → limit.context
    "max_prompt_tokens": ...,           // → limit.input
    "max_output_tokens": ...            // → limit.output
}

Impact

The under-reported limits cause tools like OpenCode to trigger context compaction 40K–136K tokens earlier than necessary, significantly degrading the user experience especially for Claude and GPT-5.2 models.

Fixes #858