feat(P01): SSH-push transport layer — connection pool, retry, fan-out, idempotent writes (REQ-073)

P01 — Load-bearing replacement for v0.8 mTLS transport (R-001).

Transport (internal/sshpush/transport.go, REQ-073):
- Transport struct with sync.Map connection pool (reuse *ssh.Client per peer).
- Exec with context timeout (10s default) + retry (100ms x2 cap 5s max 5
  attempts, +/-25% jitter — same backoff as v0.8 transport/retry.go).
- ReadFile, WriteFile (atomic heredoc + mv), Close.
- sshDialer + sshSession seams for testability. TOFU host-key verification
  reuses proxmox.TOFUHostKeyCallback. security.Flock for known_hosts.

Fan-out (internal/sshpush/fanout.go):
- ExecAll, WriteAll with errgroup + SetLimit semaphore (default 8 per I-B-001).
  Per-peer errors collected, don't cancel the group.

Idempotency (internal/sshpush/idempotency.go, C-18):
- WriteFileIdempotent: SHA-256 compare via ssh sha256sum; skip if content
  matches (written=false). Content-addressed idempotency replaces the v0.8
  X-Orca-Idempotency-Key header (C-18 capability map).

Tests: in-process fake SSH server (ssh.NewServerConn NoClientAuth ed25519)
for e2e + interface seams for pure-logic. 93.0% coverage. 20 packages pass.

---ci---
project: orca
phase: P01
milestone: v0.9
status: execute
---/ci---

internal/sshpush/doc.go

@@ -0,0 +1,23 @@
+// Package sshpush implements the v0.9 SSH-push transport layer (REQ-073,
+// R-001): the CLI on the operator host SSHes to each peer to render files,
+// apply configs, and run commands. It replaces the v0.8
+// internal/transport mTLS HTTP layer.
+//
+// The Transport reuses one *ssh.Client per peer across multiple
+// operations within a single CLI invocation (I-B-001), retries transient
+// failures with exponential backoff (100ms ×2, cap 5s, max 5 attempts —
+// reimplemented from the v0.8 transport/retry.go pattern, since
+// internal/transport is deprecated and not imported), applies per-call
+// timeouts (10s exec, 30s SCP per I-B-001), and fans out to many peers
+// with bounded concurrency (default 8, errgroup + semaphore).
+//
+// Idempotency is content-addressed (C-18): WriteFile / WriteFileIdempotent
+// compare the remote file's SHA-256 to the local content and skip the
+// write on match — the SSH-push equivalent of the v0.8 X-Orca-Idempotency-Key.
+//
+// Host-key verification reuses proxmox.TOFUHostKeyCallback (D-035), which
+// reads/writes the known_hosts file (certpaths.KnownHostsPath during the
+// v0.9 dual-write window; the move to paths.KnownHostsPath happens in
+// v0.10-P14). The known_hosts file is flock-protected inside the TOFU
+// callback, so the Transport does NOT re-lock.
+package sshpush