Add SFTP tools with SCP fallback

AGENTS.md

@@ -30,10 +30,12 @@ The first implementation pass intentionally narrows the MVP:
 * .NET 10 stdio MCP server
 * `ssh_exec`
 * `terminal_start`, `terminal_write`, `terminal_read`, and `terminal_stop`
+* `sftp_list`, `sftp_get`, and `sftp_put`
 * SSH.NET-based SSH connections
 * Tool-supplied `host`, `username`, optional `port`, optional `keyPath`, and optional `keyPassphrase`
 * Default key discovery from `~/.ssh/id_ed25519`, `~/.ssh/id_ecdsa`, then `~/.ssh/id_rsa` when `keyPath` is omitted
-* No SSH agent, OpenSSH alias, `ssh -G`, ProxyJump, ProxyCommand, or SFTP support yet
+* SFTP first for file transfer; `sftp_get` and `sftp_put` silently fall back to SCP if SFTP is unavailable
+* No SSH agent, OpenSSH alias, `ssh -G`, ProxyJump, or ProxyCommand support yet
 * Basic audit logging and timeout enforcement
 
 ## Non-Goals
@@ -169,21 +171,21 @@ Priority order:
 
 ```text
 1. SFTP
-2. SCP fallback (optional future enhancement)
+2. SCP fallback for get/put
 ```
 
 The implementation must:
 
-* Validate SFTP subsystem availability during connection
-* Return structured capability errors if unavailable
-* Not silently downgrade to SCP
+* Use SFTP for directory listing
+* Use SFTP first for file get/put
+* Silently fall back to SCP for file get/put if SFTP is unavailable
 
 Example error:
 
 ```json
 {
   "error": "sftp_unavailable",
-  "message": "Remote host does not expose the SFTP subsystem.",
+  "message": "Remote host does not expose the SFTP subsystem for directory listing.",
   "scpFallbackAvailable": false
 }
 ```
@@ -243,7 +245,6 @@ Input:
 {
   "host": "prod-api.example.com",
   "username": "deploy",
-  "shell": "bash",
   "cols": 120,
   "rows": 40,
   "port": 22,
@@ -267,6 +268,7 @@ Requirements:
 * Maintain server-side session state
 * Support idle timeout cleanup
 * Use the same key-auth inputs and default key discovery as `ssh_exec`
+* Use the remote account's default shell; do not write shell setup commands into the PTY after startup
 
 ---
 
@@ -553,6 +555,9 @@ The MVP must include:
 * terminal_write
 * terminal_read
 * terminal_stop
+* sftp_list
+* sftp_get
+* sftp_put
 * basic audit logging
 
 ---