File sync fails when coder_agent.dir is set to a non-home directory #238

Current Behavior

File sync fails to install the Mutagen agent on workspaces where the coder_agent resource has a dir property set to a directory other than $HOME.

The error:

I can see the agent was scp'd to $HOME/coder (repo location):

coder@upload-code-pastebin-dcbe:~$ grep -r mutagen-agent /home/coder/*
grep: /home/coder/coder/.mutagen-agentc8a65917-9e3a-40b6-9af9-947bfd9e27ac: binary file matches

Root Cause Investigation

This seems to be caused by an interaction between Coder Desktop's bundled Mutagen binary and the breaking change in coder/coder#21194, which made SFTP/SCP respect coder_agent.dir.

Prior to that PR, SCP landed files in $HOME while SSH sessions used dir. Mutagen's agent installation relied on SCP landing the binary in $HOME and then SSH executing it from $HOME. The original mismatch between SCP and SSH working directories was tracked in coder/coder#16568.

After the PR, both SCP and SSH now use dir. However, Mutagen appears to use an absolute $HOME-based path (/home/coder/.mutagen-agent<UUID>) when SCPing the binary, bypassing the dir setting. The SSH execution step then looks for the binary relative to dir (/home/coder/coder/), and the file isn't there.

SSHing into the workspace and running ls -la ~/.mutagen-agent* shows no files, confirming the binary never lands in $HOME.

Steps to Reproduce

1. Use a Coder deployment running v2.30+ (which includes the SFTP/SCP dir change from PR #21194)
2. Use a template that sets dir on coder_agent to a non-home directory, e.g.:

   resource "coder_agent" "dev" {
     arch = "amd64"
     os   = "linux"
     dir  = "/home/coder/coder"
   }

  (The dogfood "Write Coder on Coder" template does this)

3. Open Coder Desktop -> File Sync
4. Configure a sync session to the workspace
5. Observe the error on "Installing agent..."

Expected Behavior

Mutagen installs its agent and file sync begins successfully, regardless of the coder_agent.dir setting.

Workaround

Unsetting coder_agent.dir fully works around this issue.

Environment

• Coder Desktop: v0.8.0 (macOS)
• Coder deployment: v2.30.1+ (dogfood, running main)
• Template: dogfood "Write Coder on Coder"
   (dir = local.repo_dir -> /home/coder/coder)
• Workspace: Linux (amd64)

Additional Context

• Original SCP/SSH mismatch issue: coder_agent dir property breaks Mutagen file sync from Windows coder#16568
• Breaking SFTP/SCP change: fix(agent/agentssh)!: use configured directory for SFTP connections coder#21194