feat: prefer gh CLI over web scraping for GitHub URLs

[huaiyuWangh]

Summary

Add prompt instructions to guide agents to use gh CLI instead of web_fetch when fetching GitHub content (PRs, issues, repos, commits). This provides structured JSON data with better quality and reliability than HTML parsing.

Changes

• bash.tpl: Add <github_integration> section with gh CLI command examples for PRs, issues, repos, and commits
• agentic_fetch.md: Add tip in <tips> section to prefer gh CLI for GitHub URLs
• web_fetch.md: Add tip to use gh CLI as primary method for GitHub content

Benefits

• ✅ Better data quality with structured JSON vs fragile HTML parsing
• ✅ Access to rich metadata (reviews, check runs, reactions, timeline events)
• ✅ More reliable API contract vs HTML structure changes
• ✅ Faster direct API access vs web scraping overhead
• ✅ Graceful fallback to web_fetch if gh CLI fails

Implementation Approach

This is a prompt-driven solution with no code changes required:

• Main agent sees GitHub URL → considers bash tool (with gh CLI guidance) instead of agentic_fetch
• If gh CLI fails → gracefully falls back to web_fetch
• Easy to rollback by reverting prompt files

Test Plan

• Test with PR URL: Verified agent uses gh CLI to fetch PR data
• Test with issue URL: Verified agent uses gh CLI to fetch issue data

- [x] Verified fallback: Confirmed web_fetch is used when gh CLI unavailable

Note on Test Failures

The test failures in TestCoderAgent are expected because the bash tool prompt was modified (added <github_integration> section), which changes the system prompt sent to the API. The VCR cassettes need to be re-recorded with the new prompt.

To fix: Delete or re-record the cassette files in internal/agent/testdata/TestCoderAgent/*/ and run tests with valid API keys.

Closes #2215