feat(dao): scaffold aldabra-dao crate (Phase 1 reads)

Adds a 4th workspace crate `aldabra-dao` for native Agora-on-Cardano
DAO interaction. Multi-DAO from day one — DaoConfig per DAO at
\$ALDABRA_DATA/daos/<name>.json + .active selector. Sulkta DAO and any
community Agora deployment (Bob's DAO, Alice's DAO) are first-class.

Phase 0 type port complete:
- StakeDatum, StakeRedeemer, ProposalAction, ProposalLock, Credential
- ProposalDatum, ProposalRedeemer, ProposalStatus, ProposalThresholds,
  ProposalTimingConfig, ProposalVotes
- GovernorDatum, GovernorRedeemer
- All Constr indices verified against Agora source makeIsDataIndexed
  + EnumIsData declarations (Stake/Proposal/Governor/Action/Status all
  cross-referenced)
- Round-trip tests for every type

Phase 1 read surface (this commit):
- DaoStore: DaoConfig load/save/list/remove + active-DAO selector with
  first-register-becomes-active UX. 8 unit tests.
- DaoReader trait + KoiosDaoReader impl for get_governor + list_stakes.
  list_proposals stubbed pending Phase 4 proposal-script-address discovery.
- Stake address sharing handled: list_stakes filters on gov_token_policy
  (the shared MLabs stakes addr serves many DAOs).

Stubs for upcoming phases:
- agora/treasury.rs (Phase 4 — treasury spend helpers)
- agora/authority_token.rs (Phase 4 — GAT mint/burn)
- agora/reference_scripts.rs (Phase 2/3 — independent script-hash discovery
  per Sulkta's choice 2026-05-05; computed locally, never trust MLabs registry)
- builder/mod.rs (per-operation Plutus tx builders, populated phase-by-phase)

Spec doc + decisions: internal notes in workspace.

Effects map (`ProposalDatum.effects`) kept as raw PlutusData for round-trip
integrity until Phase 4 (proposal create) needs typed access.

ExUnits strategy locked: Koios tx_evaluate from day one (no hardcoded values).
Wired up in Phase 2 alongside reference-script discovery.

crates/aldabra-dao/src/error.rs

@@ -0,0 +1,52 @@
+//! Crate-wide error type.
+
+use thiserror::Error;
+
+/// Anything that can go wrong inside `aldabra-dao`.
+///
+/// Boundary rule: `DaoError` does NOT wrap `aldabra_core::WalletError` or
+/// `aldabra_chain::ChainError` directly — those sit at different layers.
+/// Convert to a [`DaoError::Backend`] string via the `?`-helper at call sites
+/// to keep the public error surface flat for MCP consumers.
+#[derive(Debug, Error)]
+pub enum DaoError {
+    /// Per-DAO config file is missing, malformed, or names a DAO we don't have.
+    #[error("dao config: {0}")]
+    Config(String),
+
+    /// Bech32 / address parse failure.
+    #[error("address: {0}")]
+    Address(String),
+
+    /// PlutusData encode/decode failure — the on-chain datum didn't match
+    /// the shape we expected.
+    #[error("datum: {0}")]
+    Datum(String),
+
+    /// CBOR encode/decode failure (lower-level than [`Self::Datum`]).
+    #[error("cbor: {0}")]
+    Cbor(String),
+
+    /// Backend (Koios / submit) returned an error.
+    #[error("backend: {0}")]
+    Backend(String),
+
+    /// We couldn't find a required reference-script UTxO on chain.
+    #[error("reference script not found: {0}")]
+    RefScript(String),
+
+    /// Caller asked for something the DAO can't provide right now (no active
+    /// proposal, no stake registered, locked stake, etc).
+    #[error("invalid state: {0}")]
+    State(String),
+
+    /// I/O error against the DAO config dir or similar.
+    #[error("io: {0}")]
+    Io(#[from] std::io::Error),
+
+    /// JSON serialization failure for [`crate::config::DaoConfig`].
+    #[error("json: {0}")]
+    Json(#[from] serde_json::Error),
+}
+
+pub type DaoResult<T> = Result<T, DaoError>;