feat(qrypt): qrypt.chat anonymous-invite issuer

AgentBBS becomes a trusted Ed25519 issuer for qrypt.chat anonymous
accounts. Members mint a signed, single-use qci1 token (per the shared
invite contract) that qrypt.chat verifies and redeems.

- internal/qryptinvite: Mint / GenerateIssuerKey / ParsePrivateKey +
  Config (AGENTBBS_QRYPT_* env) with unit tests (independent verify,
  payload assertions, jti uniqueness, tamper rejection, seed/full key).
- store: qrypt_invites table + QryptInviteCount / RecordQryptInvite
  (per-member quota, enforced in a tx; ErrQuotaExceeded) + test.
- plugins/qryptinvite: hub plugin (members only) — checks quota, mints,
  records, prints token + redeem URL.
- cmd/agentbbs: `qrypt-invite <user>` and `qrypt-issuer-keygen`
  subcommands wired into dispatch.
- setup.sh env template + docs/qrypt-invites.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/qrypt-invites.md

@@ -0,0 +1,105 @@
+# qrypt.chat anonymous invites
+
+AgentBBS is a **trusted issuer** for [qrypt.chat](https://qrypt.chat) anonymous
+accounts. A verified AgentBBS member can mint a signed, single-use invite token;
+the separate qrypt.chat app verifies the signature and redeems the token into an
+**anonymous** account (no phone number). AgentBBS holds the private key and
+signs; qrypt.chat only ever sees the public key.
+
+This is additive and isolated: it touches nothing in the join/SMS/pod paths.
+
+## The bridge
+
+```
+member  --ssh-->  AgentBBS (issuer, has Ed25519 priv key)  --signed token-->  qrypt.chat (verifier, has pub key)
+```
+
+- AgentBBS mints `qci1.<payload>.<sig>` tokens with `crypto/ed25519`.
+- qrypt.chat looks up the issuer's public key by `payload.iss` in its
+  `invite_issuers` table, verifies the signature, checks expiry, and burns the
+  `jti` (single use) on redeem.
+
+## Token format (v1)
+
+A single fixed algorithm (Ed25519) — not a JWT, no `alg` field.
+
+```
+token         = "qci1." + b64url(payloadJSON) + "." + b64url(sig)
+signing input = "qci1." + b64url(payloadJSON)        # the first two segments
+sig           = Ed25519.Sign(issuerPriv, []byte(signingInput))
+b64url        = base64 URL-encoding, NO padding
+```
+
+`payloadJSON`:
+
+```json
+{ "jti": "16-random-bytes-hex", "iss": "agentbbs", "tier": "anonymous",
+  "iat": 1700000000, "exp": 1700604800, "uses": 1 }
+```
+
+Implemented in [`internal/qryptinvite`](../internal/qryptinvite). Verifier rules
+(qrypt.chat side): exactly 3 segments; `segment[0] == "qci1"`; known/enabled
+issuer; valid signature; `now <= exp`; `jti` not already redeemed.
+
+## Setup (operator, once)
+
+1. Generate a keypair on the AgentBBS host:
+
+   ```bash
+   agentbbs qrypt-issuer-keygen
+   ```
+
+   It prints a **private seed** (base64) and a **public key** (base64).
+
+2. Set the private seed in `agentbbs.env` (see `setup.sh`) and restart:
+
+   ```
+   AGENTBBS_QRYPT_ISSUER_KEY=<base64 seed>
+   ```
+
+   Until this is set, minting is disabled (the plugin says "not configured").
+
+3. Register the **public** key in qrypt.chat (service-role / SQL):
+
+   ```sql
+   INSERT INTO invite_issuers (id, name, ed25519_public_key)
+     VALUES ('agentbbs', 'AgentBBS', '<base64 public key>');
+   ```
+
+   The `id` must match `AGENTBBS_QRYPT_ISSUER_ID` (default `agentbbs`).
+
+## Config (env)
+
+| Var | Default | Meaning |
+|---|---|---|
+| `AGENTBBS_QRYPT_ISSUER_ID` | `agentbbs` | issuer id; must match qrypt's `invite_issuers.id` |
+| `AGENTBBS_QRYPT_ISSUER_KEY` | unset | base64 Ed25519 seed (32B) or full key (64B); **required to mint** |
+| `AGENTBBS_QRYPT_INVITE_TTL` | `168h` | token lifetime (Go duration) |
+| `AGENTBBS_QRYPT_REDEEM_URL` | `https://qrypt.chat/anon?invite=` | redeem URL; the token is appended |
+| `AGENTBBS_QRYPT_INVITE_QUOTA` | `5` | per-member cap (0 = unlimited) |
+
+## Member usage (over SSH)
+
+From the hub, pick **"qrypt.chat invite"**:
+
+```bash
+ssh <name>@bbs.profullstack.com    # hub → "qrypt.chat invite"
+```
+
+It checks the member's quota, mints a single-use token, records it (incrementing
+the quota), and prints the redeem URL plus the raw token. The member opens the
+URL on qrypt.chat to create their anonymous account.
+
+## Ops usage (CLI)
+
+```bash
+agentbbs qrypt-invite <username>      # mint on behalf of a member (respects quota)
+agentbbs qrypt-issuer-keygen         # print a fresh seed + public key (first-time setup)
+```
+
+## Quota storage
+
+Per-member issuance is counted in the AgentBBS SQLite `qrypt_invites` table
+(`jti` PRIMARY KEY, `username`, `created_at`). `RecordQryptInvite` enforces the
+cap inside a transaction, so concurrent mints can't both exceed it. The stored
+`jti` values are also an audit trail of what AgentBBS handed out.