Update SSH connection errors documentation with root cause analysis

**Motivations:**
- Document the deep root cause of ControlSocket already exists errors
- Explain why ControlMaster=auto doesn't remove invalid sockets
- Document the solution to aggressively clean up dead sockets

**Root causes:**
- Documentation needed to explain the root cause of socket invalidation
- Need to document why directory removal is necessary for proper cleanup

**Correctifs:**
- Added deep root cause analysis section explaining ControlMaster=auto behavior
- Documented why dead sockets accumulate and cause subsequent failures
- Updated corrections section with latest improvements

**Evolutions:**
- More complete documentation of the problem and solution

**Pages affectées:**
- fixKnowledge/ssh-connection-errors-deployment.md: Added root cause analysis and updated corrections

fixKnowledge/ssh-connection-errors-deployment.md

@@ -44,6 +44,26 @@ The SSH ControlMaster feature creates a persistent connection to avoid multiple
 - Command substitution `$()` with SSH commands can block indefinitely if the connection hangs
 - The original logic used `grep` on command output, which required capturing all output and could hang
 
+### Deep Root Cause: ControlSocket Already Exists
+
+The fundamental issue is that:
+
+1. **Socket can become invalid between check and execution**: The socket file can exist on the filesystem, but the underlying connection can die between the validation check and the actual command execution.
+
+2. **ControlMaster=auto behavior**: When SSH detects that a ControlMaster socket exists but is invalid, it does NOT remove the socket. Instead, it:
+   - Disables multiplexing for that command
+   - Creates a new connection without multiplexing
+   - Leaves the dead socket file in place
+
+3. **Dead socket accumulation**: The dead socket remains and causes subsequent commands to fail with "ControlSocket already exists, disabling multiplexing" errors.
+
+4. **Insufficient cleanup**: Previous cleanup attempts only removed the socket file, but if a process was still holding it, the socket could remain. Removing the entire directory ensures complete cleanup.
+
+The solution is to:
+- Test actual connection usability (not just socket existence)
+- Aggressively remove the entire socket directory when invalid
+- Recreate the directory for fresh connections
+
 ## Corrections Applied
 
 ### 1. Enhanced SSH Connection Management
@@ -56,12 +76,17 @@ The SSH ControlMaster feature creates a persistent connection to avoid multiple
 - Enhanced `ssh_exec()` function with:
   - Connection validation before each command
   - Automatic cleanup of dead connections
-  - Retry mechanism (up to 3 attempts)
   - Additional SSH options for better connection stability:
     - `ConnectTimeout=10`: Fail fast if connection cannot be established
     - `ServerAliveInterval=60`: Keep connection alive
     - `ServerAliveCountMax=3`: Detect dead connections quickly
 
+**Latest improvements (root cause fix)**:
+- `check_ssh_connection()` now tests actual connection usability with `ssh ... true` instead of just `ssh -O check`
+- `cleanup_dead_ssh()` now removes entire socket directory instead of just socket file
+- This ensures socket is truly removed even if process is still holding it
+- Directory is recreated after cleanup for fresh connections
+
 ### 2. Improved Error Handling at Step 5
 
 **File**: `deploy.sh`