tasks: decompose vault, core, call crates into 28 atomic implementation tasks

Break down the three initial crates (alknet-vault, alknet-core, alknet-call)
into dependency-ordered task files for implementation agents.

Structure:
- tasks/vault/ (10 tasks) — drift fixes from ADR-025/026 refactor, review,
  spec sync. Vault is independent and can run fully in parallel with core/call.
- tasks/core/ (6 tasks) — crate init, core types, config, auth, endpoint,
  review. Core is foundational; call depends on it.
- tasks/call/ (12 tasks) — split into registry/ and protocol/ topic subdirs
  reflecting the two subsystems. CallAdapter is the merge point.

Key decisions:
- Drifts 3+9+10 grouped as one task (key-versioning-rotation) — the complete
  ADR-021 rotation feature that doesn't compile in pieces
- Reviews injected at end of each crate phase (vault, core, call)
- Vault spec-sync task removes the drift table and bumps doc status to stable
- ACME deferred in core/endpoint (noted as TODO; X509 manual certs for now)
- OperationEnv kept as a trait (load-bearing for ADR-024 layering)

Validated: 28 tasks, no cycles, 11 generations of parallel work.
Critical path runs through call (11 tasks). Vault completes by generation 4.
6 high-risk tasks identified (21%): irpc-removal, endpoint, operation-context,
operation-env, call-adapter, abort-cascade.

tasks/vault/unlock-new-zeroizing-return.md

@@ -0,0 +1,79 @@
+---
+id: vault/unlock-new-zeroizing-return
+name: Change unlock_new return type from String to Zeroizing<String>
+status: pending
+depends_on: [vault/irpc-removal]
+scope: single
+risk: low
+impact: isolated
+level: implementation
+---
+
+## Description
+
+Fix drift item #8: `unlock_new` currently returns `String`, which is not
+zeroized on drop. The mnemonic phrase is the root of trust — it must not linger
+in freed heap memory. Change the return type to `Zeroizing<String>` (from the
+`zeroize` crate, already a dependency).
+
+### Current state
+
+```rust
+pub fn unlock_new(&self, word_count: usize) -> Result<String, VaultServiceError>;
+```
+
+### Target state
+
+```rust
+pub fn unlock_new(&self, word_count: usize) -> Result<Zeroizing<String>, VaultServiceError>;
+```
+
+Per `docs/architecture/crates/vault/service.md` → unlock_new:
+
+> The returned phrase is the root of trust — it is heap-allocated and zeroized
+> on drop, so it does not linger in freed memory. The caller should extract the
+> phrase for secure storage (write down, display to user) and let the
+> `Zeroizing<String>` drop when done. Do not clone the returned value or store
+> it in a non-zeroizing container.
+
+### Caller adaptation
+
+The assembly layer (CLI binary, not yet implemented) will call `unlock_new` and
+extract the phrase. The `Zeroizing<String>` wrapper derefs to `String`, so
+`&*result` or `result.as_str()` works for reading. The caller must not clone the
+inner `String` into a non-zeroizing container.
+
+Existing tests that call `unlock_new` need updating to handle the new return
+type — use `&*phrase` or `phrase.as_str()` to read the string.
+
+### Scope
+
+This task touches `service.rs` (the method signature and body) and test files.
+It depends on the irpc removal task (drift #4) because both modify `service.rs`.
+
+## Acceptance Criteria
+
+- [ ] `unlock_new` return type changed from `Result<String, ...>` to `Result<Zeroizing<String>, ...>`
+- [ ] Method body constructs `Zeroizing<String>` from the generated phrase
+- [ ] Existing tests updated to handle `Zeroizing<String>` return type
+- [ ] No `clone()` of the returned value in non-test code
+- [ ] `cargo check` succeeds
+- [ ] `cargo test` succeeds
+- [ ] `cargo clippy` succeeds with no warnings
+
+## References
+
+- docs/architecture/crates/vault/README.md — Known Source Drift table item #8
+- docs/architecture/crates/vault/service.md — unlock_new section
+- docs/architecture/decisions/025-vault-local-only-dispatch.md — ADR-025 (resolves W7)
+
+## Notes
+
+> The mnemonic is the root of trust. Returning a plain `String` means the phrase
+> lingers in freed heap memory after the caller drops it. `Zeroizing<String>`
+> zeroizes the bytes on drop. This resolves review #002 W7. Depends on irpc
+> removal because both modify `service.rs`.
+
+## Summary
+
+> To be filled on completion