feat(memory): adopt mimalloc allocator

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Purpose

Replace the platform allocator-specific telemetry and reclamation paths with one cross-platform mimalloc backend. This PR stacks on cortexkit#283.

Changes

• install mimalloc as the Rust global allocator in production and allocation-counted tests
• derive allocator requested, committed, and retained-byte telemetry from mimalloc statistics
• replace mallinfo2, malloc_trim, and macOS zone relief with forced mi_collect
• retain the detached aft-mem-relief thread and the existing transport-thread cadence gate
• trigger forced collection after idle artifact eviction
• report reclaimed residency using allocator commitment or observed RSS reduction
• document the allocator backend and collection flow

Verification

• allocator contracts: 9 passed
• forced warm-then-idle experiment reclaimed 64,024,576 bytes of RSS
• cargo check -p agent-file-tools --all-targets
• cargo fmt --all -- --check
• git diff --check

Scope

This PR changes allocator ownership and reclamation only. Explicit subsystem budgets, charge-before-build admission, and cache eviction policy remain separate work.

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Superseded by cortexkit#285.