fix(dx): Improve error messages when Ollama is not running

[sunilp]

Summary

When Ollama is not running or unreachable, the current error message could be more helpful.

Current Behavior

The error message mentions that Ollama is unreachable but could provide more actionable guidance.

Expected Behavior

The error should:

1. Clearly state that Ollama is not reachable at the configured URL
2. Suggest running ollama serve to start it
3. If it's a connection refused error, suggest checking if Ollama is installed
4. If using a non-default URL, mention the configured baseUrl
5. Suggest running jam doctor for full diagnostics

Files to Check

• src/providers/ollama.ts — error handling in validateCredentials(), streamCompletion(), and listModels()
• src/utils/errors.ts — JamError class

Acceptance Criteria

• Clear, actionable error messages for connection refused / timeout / DNS failure
• jam doctor suggestion in error output
• No regression in existing tests

[sunilp]

✅ Implemented and verified — closing

Merged in #11 (commit 1c908d9).

Delivered (changes to src/providers/ollama.ts):

• Detects ECONNREFUSED / ECONNRESET error codes for precise messaging
• When connection refused: "Ollama is not running at <url>. Start it with: ollama serve\nIf Ollama is not installed, visit: https://ollama.com\nRun jam doctor for full diagnostics."
• Other network failures: "Cannot reach Ollama at <url>. Start it with: ollama serve\nRun jam doctor for full diagnostics."
• HTTP error responses also include the jam doctor hint
• Applied to both validateCredentials() and streamCompletion() catch blocks

Test results — 6/6 ollama tests pass, 0 regressions (npm test, 160 total):

✓ src/providers/ollama.test.ts (6)