fix(pack-git): report effective digest budget

[ohdearquant]

Summary

• report both requested and effective max_items values from git.digest
• add a warning whenever the requested budget is clamped to the supported range
• cover both lower- and upper-bound clamping in acceptance tests

Verification

• cargo fmt -p khive-pack-git -- --check
• cargo clippy -p khive-pack-git --all-targets -- -D warnings
• cargo test -p khive-pack-git

Regression tests:

• digest_verb_max_items_negative_and_zero_clamp_to_one
• digest_verb_sources_stopped_early_on_budget_exhaustion

Fixes #2074