fix(agent): default process working directory to agent dir or $HOME

[mafredri]

Processes started via the agent process API inherited the agent's own working directory (/tmp/coder.xxx) when no WorkDir was specified. SSH sessions already use a fallback chain in agent/agentssh/agentssh.go:898-910: configured agent directory > $HOME. The process manager had no equivalent.

This wires the same manifest directory closure into agentproc.NewAPI so the resolved working directory priority is:

1. Explicit req.WorkDir from the request
2. Agent's configured directory (from manifest, validated with os.Stat)
3. $HOME

The resolved directory is recorded on the process struct so ProcessInfo.WorkDir and pathStore git notifications reflect where the process actually ran, not just what was in the request.

🤖 This PR was created with the help of Coder Agents, and has been reviewed by a human. 🏂🏻

[kylecarbs]

Ahh, much nicer! Good find

[mafredri]

Actually, going to pipe through configured working directory from coder agent.

[johnstcn]

How often is this likely to change during an agent lifecycle?
Could this just be a string?

[mafredri]

It needs to be a function. The processAPI is created once in agent.init() (line 388), but the manifest directory can change across reconnects since run() calls a.manifest.Swap() (line 1262) each time. A plain string would capture whatever value the manifest had at init time (empty, since the manifest has not been fetched yet).

The closure reads a.manifest.Load().Directory at process-start time, picking up the latest value. This is the same pattern SSH uses for WorkingDirectory at line 334.

The nil guard on the manifest (if m := a.manifest.Load(); m != nil) is the one difference from SSH, which dereferences without checking. The guard is there because processAPI is created in init() before run() populates the manifest. In practice the HTTP routes are unreachable until the manifest is loaded (the tailnet listener starts after manifestOK), but the guard costs nothing.

🤖 I traced the init path so you don't have to.