fix(milestones): remove released issues from the allocations map

[yggdrasilworld]

What — phantom ledger drift

release_issue marked the issue Released + paid out, but left the entry in milestone.allocations — the map grew into a phantom ledger where 'allocated' no longer meant outstanding.

Fix

Remove the entry on release. (remaining_budget was already reduced at allocate-time, so it's correctly untouched.)

Resolves #608

Payment: Base USDC 0xD40Df5c27cEa7dd78A3c345401995c319589334e

[vercel]

@yggdrasilworld is attempting to deploy a commit to the chonilius' projects Team on Vercel.

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[yggdrasilworld]

Verification note

The touched Rust sources parse cleanly — balanced structure, no duplicate functions, no merge artifacts. cargo test couldn't run locally: the repo pins toolchain 1.82.0 + wasm32v1-none (unavailable for that channel) and the crates declare rust-version = 1.95.0; my sandbox's linker is also blocked pending an Xcode license. Diff-level verification: release_issue removes the released entry from milestone.allocations. The contracts/*/test.rs additions are included in the diff; the suite will exercise them under the repo's CI.