Document a minimum supported Git version (blob:limit filter is version-sensitive)

[beardthelion]

Problem

clone_repo() in crates/gitlawb-node/src/sync.rs runs git clone --mirror --filter=blob:limit=10g in promisor mode. The blob:limit filter syntax is sensitive to the local Git version, and the repo pins no minimum Git version anywhere (README, docs/RUN-A-NODE.md, or CI). CI only ever exercises it against ubuntu-latest's Git, so any version skew is invisible to the test suite.

Surfaced during review of #67: jatmn's Git 2.53 (Windows) rejected the --filter=blob:limit=10g invocation, while ubuntu-latest accepts it. The failure is unrelated to #67's diff, but it exposed a real portability gap.

What to do

• Determine the actual minimum Git version that supports the --mirror --filter=blob:limit=<size> form we rely on, and confirm the size-suffix (10g) parsing floor.
• Document that minimum in README / docs/RUN-A-NODE.md as a node runtime requirement.
• Consider a startup or CI check that verifies the local Git meets the floor, so a too-old (or differently-behaving) Git fails loudly instead of at clone time.

Notes

• Code: crates/gitlawb-node/src/sync.rs (clone_repo, promisor branch).
• Last touched by Peer sync downgrades still-withheld mirrors to Plain on a transient withheld-paths error #48/fix(node): preserve promisor mirror mode on unknown withheld-paths lookup (#48) #69 and feat(node): blind recipient identities at rest and gate B1 by repo readability #40; not introduced by fix(node): fail closed when a recipient DID can't be resolved (#47) #67.