feat: add URL links to all GitHub event notifications

[IYENTeam]

Summary

Add clickable URL links to all GitHub event notifications that were missing them.

Problem

PR status-changed and issue events (opened/commented/closed) were rendered without links, making it impossible to click through to the actual GitHub page from Discord notifications. CI events already had URLs.

Changes

File	What
src/source/github.rs	Add url field to IssueSnapshot, html_url to GitHubIssue struct, pass through to events
src/events.rs	Add url: String parameter to github_issue_opened/commented/closed constructors
src/render/default.rs	Append URL to PR and issue event messages in Compact/Alert/Inline formats
src/daemon.rs	Extract /issue/html_url from webhook payload
src/cli.rs	Add --url flag to IssueOpened CLI command
src/main.rs	Pass url through CLI dispatch
src/router.rs	Update test fixtures

Before/After

# Before
PR IYENTeam/Hent-ai#10 open -> merged: docs: add onboarding section to READMEs

# After
PR IYENTeam/Hent-ai#10 open -> merged: docs: add onboarding section to READMEs · https://github.com/IYENTeam/Hent-ai/pull/10

Verification

• cargo check: ✅ clean
• cargo test: ✅ all passed
• CI events: already had URLs, no change needed