fix: preserve provider errors in user profile learning

[lindixu6-hash]

Summary

When the opencode provider path in analyzeUserProfile() throws and no manual external API fallback (memoryModel/memoryApiUrl) is configured, the original provider error was silently replaced by a generic "External API not configured for user memory learning" message. This made debugging provider failures (e.g. unsupported tool_choice in thinking mode) impossible from logs.

Changes

• Store the opencode provider error in a local variable (same pattern as fix: preserve opencode auto-capture errors #258 for auto-capture).
• When no manual fallback is configured, rethrow the original error instead of the generic message.
• When a manual fallback IS configured, behavior is unchanged: the error is logged and the fallback runs.
• Added regression test user-profile-learning-error.test.ts that mocks a throwing opencode provider with no fallback and verifies the surfaced error contains the real root cause.

Testing

• bun test tests/user-profile-learning-error.test.ts — 1/1 pass
• tsc --noEmit clean

Closes #265

[lindixu6-hash]

Reviewer quick guide: This is a 3-line source change mirroring the exact pattern already merged in #258 (auto-capture error preservation, reviewed by @NaNomicon).

Source diff (src/services/user-memory-learning.ts):

1. Declare opencodeProviderError before the try block (line 635)
2. Capture the caught error (line 698)
3. Rethrow it when no manual fallback exists (lines 706-708)

Test: tests/user-profile-learning-error.test.ts uses the same subprocess mock pattern as #258's test — mocks a throwing generateStructuredOutput, runs performUserProfileLearning, and asserts the surfaced error contains the real provider message ("Thinking mode does not support") rather than the generic "External API not configured".

Risk: Minimal — only changes the error message surfaced to users when their primary provider is broken and no fallback exists. The happy path and fallback path are untouched.