fix: Standardize webhook secret configuration across documentation

[neil-pozetroninc]

Summary

• Fixes critical configuration mismatch causing webhook validation failures
• Documentation was instructing users to set PROBOTSHARP_GITHUB_WEBHOOKSECRET but code expects PROBOTSHARP_WEBHOOK_SECRET
• This is a documentation-only change; the code already supports the correct configuration through its fallback chain

Changes

• Updated all .env.example files to use PROBOTSHARP_WEBHOOK_SECRET
• Fixed docker-compose.yml environment variable mapping
• Updated documentation in Operations.md and README files
• Standardized configuration pattern across entire codebase (7 files total)

Impact

This fixes a critical issue where users following the documentation would encounter HTTP 500 errors with "Webhook secret is not configured" when GitHub sends webhook events, even though they had configured the secret according to the documentation.

Test Plan

• All tests pass (1,058 tests passing, 0 failures)
• Build succeeds without warnings related to these changes
• Verified correct environment variable name in:
  • Root .env.example
  • Template .env.example files
  • Example bot .env.example files
  • Docker compose configuration
  • Documentation files
• Confirmed code reads PROBOTSHARP_WEBHOOK_SECRET in fallback chain

Breaking Changes

None - this is a documentation-only fix that aligns documentation with existing code behavior.