feat(agents): install manifest endpoint + slug-based install

[buremba]

Summary

Replaces the env-var leak in the closed PRs (#361 + lobu-ai/owletto-web#19). Frontend now talks slugs only; the backend resolves internal IDs.

```
GET /api/install/manifest/:slug
→ { slug, name, description, botPhone, templateAgentId }

POST /api/install
body now accepts { slug } in addition to { templateAgentId }
```

Resolution

• `slug` equals the template org's slug. We pick the canonical template agent in that org (`agents.template_agent_id IS NULL`).
• `botPhone` comes from a per-slug server env var (e.g. `PERSONAL_FINANCE_BOT_PHONE`). Stripped to bare digits so the landing page plugs straight into `wa.me/`. Null when unset.

Public, no auth — manifests are marketing data. The install POST still requires a session.

What this kills

• Build-time `VITE_PERSONAL_FINANCE_TEMPLATE_AGENT_ID` (frontend no longer needs an agent ID).
• Build-time `VITE_PERSONAL_FINANCE_BOT_PHONE` (frontend no longer needs the bot phone).
• The HMAC install-token + chat-claim flow from the closed feat(agents): install-token + chat-claim primitives for WhatsApp deep-link install #361.

Net change against the closed work: drop ~430 LOC, replace 1 page, same product.

Adding a new productized template is now:

1. Seed the org + canonical template agent (existing tooling).
2. Set a per-slug bot phone env var if you want a wa.me link.
3. Link to `/install/`.

Stacked on

`feat/install-identity-provisioning` (#359).

Test plan

• Manifest 200 returns expected shape.
• Manifest 404 for unknown slug.
• Manifest skips orgs that only contain instance agents (template_agent_id IS NOT NULL).
• Manifest returns `botPhone` as bare digits when env var is set; null when unset.
• Pre-commit Biome + tsc pass.
• Manual: `POST /api/install -d '{"slug":"personal-finance","whatsapp_phone":"+447123456789"}'` works without templateAgentId.

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[buremba]

Heads up: per docs/plans/world-model.md (on docs/world-model-plan), the env-var PERSONAL_FINANCE_BOT_PHONE slug map in install-manifest-routes.ts should go away. Bot phone becomes a metadata field on the agent_template entity (templates become entities of type agent_template in a public_catalog org). Manifest endpoint reads it from the entity, not from a hardcoded slug→env table.

The manifest endpoint itself may still be worth keeping for unauthenticated landing-page rendering, just sourced from data rather than env.