fix: add Gemini API key to credential redaction and add missing unicode regex flags

[Semianchuk Vitalii (fix2015)]

Summary

• Add GEMINI_API_KEY to credential redaction in sanitizeDiagnosticText (src/diagnostics.ts). The constant GEMINI_API_KEY_ENV_KEY already existed in constants.ts but was missing from the redaction list, meaning Gemini API keys could leak into diagnostic output.
• Fix missing punctuation in the OAuth completion HTML page (src/agent/openai-chatgpt-oauth.ts). The sentence "OpenWiki login complete you can close this tab." was a run-on; added a period and capitalized accordingly.
• Add unicode (u) flag to regex patterns in src/telemetry/gates.ts and src/telemetry/senders.ts for consistency with the rest of the codebase and correct handling of unicode input.

Test plan

• Verify sanitizeDiagnosticText redacts GEMINI_API_KEY values from diagnostic output
• Confirm OAuth completion page renders correct punctuation
• Confirm regex replacements in telemetry code behave identically for ASCII input and correctly for unicode input

[Colin Francis (colifran)]

Thanks Semianchuk Vitalii (@fix2015)!