behavior: real notifier for help-wanted-roles (replace LoggingNotifier stub)

[themightychris]

Gap

specs/behaviors/help-wanted-roles.md and specs/api/projects-help-wanted.md describe notifications going out when someone expresses interest in a help-wanted role.

Today: LoggingNotifier is a no-op stub that logs the intended notification and returns delivered: true without actually sending anything. Users expressing interest never trigger a real message to the role's contacts.

Scope (decision-required)

A single notifier interface, one implementation. Pick one:

• Email-only (likely simplest first cut) — SMTP via an existing provider (mailgun/postmark/SES). Person record's email comes from the private profile bucket.
• Slack DM — uses the existing SAML IdP credential trust to map Person → Slack user; sends DM via Slack Web API. No email leakage.
• Both — email is fallback when Slack DM fails or recipient doesn't have a linked Slack account.

Pre-implementation decision needs an owner-call on:

1. Which channel(s)
2. Where credentials live (sealed secret in cluster repo)
3. Templating shape (a few hardcoded templates is fine; no full templating engine needed)

Test plan

• Unit: notifier interface contract; mock impl
• Integration: real notification delivery against a sandbox inbox / sandbox Slack channel before cutover

Why this matters at cutover

Help-wanted-roles is in the v1 surface. Silent acceptance with no follow-up message is a noticeably worse UX than the current laddr workflow (which does email out).