D46 — Per-KB git identity (author/committer + SSH): per-KB env wins over the process, default committer = author

Amended by D904: the per-KB author and committer are removed (the writer is the author, the server the committer); the per-KB SSH key and credentials still win over the process.

Amended by D905: the per-KB SSH key is <kb>.ssh in the credentials directory (kbs[].ssh_key and kbs[].known_hosts are removed); it still wins over the process.

Decision. Connects the identity fields of KBSpec/GitConfig (D44) to the actual git commands. internal/gitx.runGitEnv(dir, env, args...) runs git with env winning over the process on duplicate keys; Clone/Commit/Fetch/Push/PullRebaseAutostash take a variadic env ...string. kb.KB gains GitAuthorName/GitAuthorEmail/GitEnv, used by CommitOp/ SyncIn/SyncOut/conflict resolution. gitEnvForKB assembles GIT_SSH_COMMAND and GIT_COMMITTER_* with cascading fallback: spec → global → default (default committer = author). Rationale. The "per-KB env wins over the process" inversion is deliberately opposite to setupGitSSH's "the environment wins" rule (global SSH fallback): at the per-KB level the identity is an explicit config choice and must prevail over an inherited process environment that could carry the wrong identity. The cascading fallback (KB → global → hardcoded) keeps zero-value = pre-existing behavior on unchanged deployments. Details: docs/transport-auth.md, docs/concurrency.md §Git synchronization to the remote.