Refactor context window calcs

[gherlein]

Summary

• The context window display ("X / Yk Z%") was hardcoded via a model-name prefix map. For Ollama models
  this was often wrong — e.g., all qwen* models were hardcoded to 4 096 tokens regardless of actual
  configuration.
• Adds OllamaContextWindowSize() to the provider package, which calls the Ollama /api/show endpoint
  at startup and reads the real value: num_ctx from the Parameters field (user/modelfile-configured
  window) or *.context_length from ModelInfo (native model capacity, works across llama, qwen2, gemma,
  etc.).
• Both CLI and ACP server startup paths now query Ollama for the actual size and fall back to the
  hardcoded map if the query fails.

Test plan

• [X ] Start pi with an Ollama model (e.g., ollama/qwen2.5-coder:32b) and confirm the context bar shows
  the correct window size instead of 4.1k.
• go test ./internal/provider/... passes.

[codecov]

Codecov Report

❌ Patch coverage is 5.55556% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.85%. Comparing base (da7a08f) to head (455344f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/provider/ollama.go	0.00%	26 Missing ⚠️
internal/acp/server/runtime.go	0.00%	5 Missing ⚠️
internal/cli/cli.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   80.96%   80.85%   -0.12%     
==========================================
  Files         174      174              
  Lines       23564    23598      +34     
==========================================
  Hits        19079    19079              
- Misses       3367     3400      +33     
- Partials     1118     1119       +1     

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