[Quality][High] Add treasury withdrawal caps and timelocked execution

[Baskarayelu]

Objective

Add a proposal/approval/execution flow with amount caps, destination validation, and a timelock.

Why this matters

Treasury withdrawals are high-impact operations that need bounded authority, review time, and replay-safe execution.

This is a substantive production-quality improvement. It must change runtime behavior, security guarantees, correctness, reliability, or meaningful user functionality. It is not a documentation-only, formatting-only, or trivial dependency task.

Scope

Area: treasury controls

Starting points: docs/treasury.md, docs/budget-ceilings.md, credence_treasury crate

The contributor should verify the current implementation before changing it and keep the PR limited to this issue. Do not introduce unrelated refactors or weaken existing CI/security gates.

Acceptance criteria

• Withdrawals require the configured approval policy and cannot exceed the active ceiling.
• A proposal binds token, amount, destination, nonce, and expiry.
• Execution before the delay, after expiry, or twice is rejected.
• Failed transfer leaves the proposal executable or cancellable according to the documented state machine.

Required validation

• Cover approval quorum, cap boundaries, expiry, cancellation, and replay.

• Test invalid destination/token and failed transfer rollback.

• Verify events support complete treasury reconciliation.

• The PR explains the failure mode, the chosen design, backward-compatibility impact, and rollback or migration considerations.

• The PR includes CI evidence and does not contain secrets, generated noise, unrelated cleanup, or disabled checks.

Contributor deliverables

• Open a focused feature branch and do not begin implementation until assigned.
• Reference this issue with Closes #<issue-number> or Fixes #<issue-number>.
• Check off every acceptance criterion in the PR with links to the relevant code and tests.
• Include a security/correctness note explaining why adversarial inputs cannot bypass the new guarantee.

Maintainer quality bar

The PR must be independently reviewable, preserve existing behavior outside this scope, exercise failure paths, and pass the repository's complete required CI/CD checks. Reward eligibility is not guaranteed by this issue or by merging.

[DevScoopee]

I haves reviewed the description and understood what needs to be done and can deliver. Kindly assign