fix(git-credential-nostr): accept BUZZ_PRIVATE_KEY with NOSTR_PRIVATE_KEY fallback

[santhiprakash]

Summary

git-credential-nostr was the only Buzz component that did not read BUZZ_PRIVATE_KEY. Users who exported the ecosystem-standard env var got silent wrong-identity signing because the helper fell back to nostr.keyfile (or no credentials) instead.

This change:

• Resolves identity in this order: $NOSTR_PRIVATE_KEY, $BUZZ_PRIVATE_KEY, then git config nostr.keyfile.
• Warns on stderr when both env vars are set to different values and $NOSTR_PRIVATE_KEY wins, so the silent wrong-identity trap is at least audible.
• Updates the helper README and docs/remote-agents.md to document the precedence.
• Adds integration tests for BUZZ_PRIVATE_KEY-only and the conflict-warning path.

Related issue

Fixes #4712.

No duplicate PR found (searched open/closed PRs for 4712, BUZZ_PRIVATE_KEY, and git-credential-nostr).

Testing

Targeted package-level verification:

source bin/activate-hermit
cargo fmt -p git-credential-nostr -- --check
cargo clippy -p git-credential-nostr --all-targets --all-features -- -D warnings
cargo test -p git-credential-nostr

All passed:

• cargo fmt — clean
• cargo clippy — clean (-D warnings)
• cargo test -p git-credential-nostr — 10 integration tests passed (including the two new regression tests)

Full just ci was not run because this is an isolated Rust crate change; the targeted tests above exercise the modified surface.