fix(admission): require explicit resource overcommit

[rldyourmnd]

Summary

• default undeclared CPU and memory reservations to the pool hard limits
• remove the implicit global memory-size overcommit table
• preserve smaller reservations only when a pool declares them explicitly
• make per-member placement account full limits by default
• update the portable queue example and regression coverage

Root cause

Private platform pools declared no reservation, so EffectiveReservation silently converted 4 GiB workers to 2.5 GiB and 6 GiB workers to 2 GiB. Queue and placement therefore admitted materially more memory than the reviewed pool contracts while natural Java, Node, CodeQL, and image jobs produced repeated host OOM.

Validation

• red regression test observed against the implicit table
• make verify

Partial fix for #183; live rollout and natural no-OOM acceptance remain private-estate work.