feat(smart-contract): add agent governance and voting mechanism

[nasarajoe]

Summary

Implements an on-chain governance system where registered agents vote on
protocol changes, parameter updates, and agent disputes. Adds a new
agent_governance Soroban contract crate.

Closes #253

What's implemented

New crate: smart-contracts/contracts/agent_governance/

File	Contents
src/lib.rs	Governance contract logic
src/types.rs	Proposal, vote, and agent-stake types + event payloads
src/errors.rs	16 stable #[contracterror] codes
src/test.rs	23 unit tests covering the full lifecycle

smart-contracts/Cargo.toml gains contracts/agent_governance as a
workspace member.

Functionality

• create_proposal — registered-agent–gated; three proposal types
  (ParameterChange, AgentDispute, ProtocolUpgrade); configurable
  voting period (default 7 days, bounded 1 h–30 days); snapshots the
  electorate's total voting power into the proposal so quorum has a
  stable denominator.
• Voting power — weighted by both reputation score and stake amount:
  voting_power = stake + reputation * REPUTATION_POWER_UNIT. Tracked in
  aggregate across all registered agents and cached per agent.
• register_agent / update_agent — agents join the electorate with
  a reputation score ([0, 100]) and stake; aggregate power is kept in
  sync on updates.
• vote_on_proposal — options For, Against, Abstain; one vote
  per agent, weighted by current voting power; rejects votes after the
  deadline or on non-active proposals.
• execute_proposal — callable by anyone after the deadline.
  • Quorum: cast power (For + Against + Abstain) ≥ 30 % of the
    snapshotted total voting power.
  • Majority: For power strictly > 50 % of decisive power
    (For + Against; abstentions excluded). Zero decisive votes → fails.
  • Marks the proposal Executed or Failed accordingly.
• View functions — get_proposal, get_vote, get_agent,
  get_total_voting_power, get_proposal_count.

Events

ProposalCreated, VoteCast, ProposalExecuted, ProposalFailed
(plus agent_reg / agent_upd), all under the gov topic namespace.

Testing

• cargo test -p agent-governance — 23 passed, 0 failed
  • proposal lifecycle across all three proposal types
  • voting-power weighting by stake + reputation
  • quorum failure (low turnout), majority failure (exact 50 % tie)
  • abstain counts toward quorum but not majority; all-abstain fails
  • double-vote, double-execute, vote-after-finalize, unregistered-voter guards
  • event emission on vote and execution
• cargo clippy -p agent-governance --all-targets — clean, no warnings
• cargo fmt applied

Acceptance criteria

• create_proposal function for governance proposals
• Voting power weighted by reputation score and stake amount
• vote_on_proposal with options: For, Against, Abstain
• Proposal types: parameter_change, agent_dispute, protocol_upgrade
• Quorum requirement: minimum 30% of total voting power
• Majority requirement: >50% of cast votes (excluding abstentions)
• Time-limited voting periods (configurable, default 7 days)
• Events: ProposalCreated, VoteCast, ProposalExecuted, ProposalFailed
• Unit tests for proposal lifecycle, voting, and execution

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