screens: fill spec'd render gaps across detail screens, Home, and ComingSoon pages

[themightychris]

Gap

Multiple spec'd screens render less than their spec describes. Audit (2026-05-19) surfaced these in a single pass:

PersonDetail

• email not in response or screen for self/staff callers (specs/screens/person-detail.md)
• slackHandle not surfaced — should show a "DM on Slack" link in the Contact sidebar (same spec)

ProjectDetail

• "Share" sidebar (copy link + "Share to Slack" button) missing (specs/screens/project-detail.md)
• "Edit on GitHub" footer link when developersUrl is a github.com URL (same spec)
• "What does this stage mean?" modal linking to stage descriptions (specs/behaviors/project-stages.md)

Home

• "Start a Project" card always links externally; should route signed-in users to /projects/create (specs/screens/home.md)

ComingSoon pages

• /projects/:slug/buzz/new renders <ComingSoon />; spec is a full create form (specs/api/projects-buzz.md)
• /pages/:slug (Mission, Leadership, Code of Conduct, Hackathons) all <ComingSoon /> — apps/web/src/content/pages/ directory doesn't exist; spec is markdown files rendered server-side (specs/behaviors/app-shell.md)
• /contact is <ComingSoon />; spec says minimum is a mailto: link (same spec)

Scope

Sub-tasks are listable; could split into per-screen PRs if convenient. Keeping as one umbrella so the spec→screen audit-and-close pass happens together.

Out of scope

• AppShell-level a11y (skip-link) and search typeahead — separate concerns, will live on the misc-bugs branch or as separate issues
• The full blog reader (#H is its own issue)