feat(report): add Google Workspace and Slack Markdown integrations

[wseaton]

Summary

• add Google Docs and Sheets workflow sources and report destinations
• add explicit, escaped Slack Markdown output and template variables
• wire the FIPS watch report to its bounded Markdown output
• acknowledge the intentional reqwest 0.12/0.13 split introduced by the Google Workspace client

Verification

• cargo fmt --all -- --check
• cargo test --workspace --no-fail-fast
• dependency guardrails
• govctl check