feat(signer): migrate Nostr publishing off account.prvkey → resolve_for_wallet (#3)

Closes aiolabs/tasks#3. Pre-cascade prerequisite for aiolabs/lnbits#17
(signer abstraction phase 1), which lands an m002 startup job that
NULLs the legacy `accounts.prvkey` column. After this migration, the
tasks extension reads no plaintext nsec and works with any
NostrSigner backend (LocalSigner / RemoteBunkerSigner / ClientSideOnlySigner).

## What changed

### nostr_hooks.py — three publisher entry points

Was: `_account_keys(wallet_id)` helper pulled `(account.pubkey,
account.prvkey)` from the wallet's owning account, returned None when
prvkey was missing, then passed both to the publishers.

Now: each of `publish_or_delete_task_event`,
`publish_task_completion`, and `publish_completion_delete` calls
`await resolve_for_wallet(...)` (the DRY helper from
aiolabs/lnbits#23 — wallet → account → signer → can_sign-check in
one call, returns None on any soft-fail). The resolved `NostrSigner`
is passed to the publisher. Soft-skip on None (wallet missing,
account unclassified, or ClientSideOnlySigner where the server has
no signing authority).

Removed the `_account_keys` helper entirely.

### nostr_publisher.py — three publishers

Was: `publish_task_to_nostr`, `publish_completion_to_nostr`, and
`publish_completion_delete_to_nostr` each accepted
`(account_pubkey: str, account_prvkey: str)` and signed via a local
`sign_nostr_event` helper that called `coincurve.PrivateKey
.sign_schnorr` directly on the plaintext nsec.

Now: each publisher accepts `signer: NostrSigner`. Signing is
factored into a shared `_sign_and_publish` helper that builds the
unsigned event dict (`kind`/`created_at`/`tags`/`content`), hands it
to `await signer.sign_event(...)`, and writes `id`/`pubkey`/`sig`
back onto the local `NostrEvent` model before publishing. The signer
backend (LocalSigner / RemoteBunkerSigner) is transparent.

Removed the `sign_nostr_event` helper entirely — the signer
abstraction handles all signing now.

Dropped the `coincurve` import; no direct crypto in this extension.

## Acceptance

- [x] `_account_keys` helper removed (nostr_hooks no longer touches account.prvkey)
- [x] all three publishers accept NostrSigner instead of (pubkey, prvkey)
- [x] extension-local Schnorr code removed (sign_nostr_event gone)
- [x] coincurve import dropped
- [x] re-grep `tasks/`: zero `account.prvkey` references
- [x] version bumped: 0.0.1 → 0.0.2 (catalog entry deferred until cascade lands)

Manual smoke testing + tag + catalog entry follow the migration
landing; will run against the regtest stack with lnbits on
`issue-18-phase-2.3` (which validates both LocalSigner and
RemoteBunkerSigner signing paths end-to-end).

## Cross-references

- aiolabs/tasks#3 — issue this commit closes
- aiolabs/lnbits#17 — the cascading signer-abstraction PR
- aiolabs/lnbits#23 — the resolve_for_wallet helper this uses
- aiolabs/lnbits#21 — umbrella audit (5 affected extensions)
- aiolabs/events#23 — sister migration (already on signer-abstraction branch)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config.json

@@ -1,6 +1,6 @@
 {
   "id": "tasks",
-  "version": "0.0.1",
+  "version": "0.0.2",
   "name": "Tasks",
   "repo": "https://git.atitlan.io/aiolabs/tasks",
   "short_description": "Recurring tasks and chore-tracking, published over Nostr",