Fix HTTP push pack negotiation memory

[mizchi]

Summary

• discover remote receive refs before building HTTP push packs
• exclude objects reachable from advertised remote refs, including nested trees and shared blobs
• skip object loading and pack upload entirely when the destination ref is already up to date
• apply the same negotiation behavior to hub notes pushes used by agent communication
• add a smart HTTP regression test for request size and no-op pushes

Root cause

HTTP push collected and delta-compressed every object reachable from local HEAD before discovering what the receiver already owned. On the reported repository that meant processing roughly 13 GB of reachable blob data even though only about 1.55 MB was missing. The existing exclusion walker also marked nested tree IDs before recursing, so shared objects under those trees were not excluded correctly.

Impact

A controlled 16 MiB fixture reduced peak RSS from 246.4 MB to 13.0 MB and the receive-pack request from 16.78 MB to 416 bytes. An up-to-date push in the original large repository now completes in 0.50 s with 39.7 MB maximum RSS.

Validation

• pkf run check
• native bit_lib and bit_lib/native tests: 315 passed
• native hub and hub/native tests
• t/t0020-push-fetch-pull.sh: 21 passed
• t/t9003-fetch-up-to-date.sh: 8 passed
• t/t0012-hub-sync.sh: passed
• t/t9020-http-push-negotiation.sh: 5 passed
• native release build
• real no-op push against the original GitHub branch