agent: log when a configured working directory is rejected and falls back to home

## Problem

`usershell.ResolveWorkingDir` (introduced in coder/coder#26102) uses a configured working directory when it exists as a directory, and otherwise silently falls back to the user's home directory. The callers (the `agentssh` SSH command and SFTP paths, and `agentproc`) do not distinguish "used the configured directory" from "fell back to home," so a rejected configured directory produces no log or metric.

Scenario: an admin sets the agent directory to a path that does not exist or is not accessible (bad mount, wrong permissions). SSH and SFTP sessions silently land in `$HOME`. Nothing correlates the rejected directory with the session, so support cannot explain why sessions start in the wrong place.

This is pre-existing behavior. All three resolvers fell back silently before the unification, which preserved the contract rather than introducing it.

## How to verify

Read `usershell.ResolveWorkingDir` and the two call sites (`agentssh.Server.resolveWorkingDir` and `agentproc.manager.resolveWorkingDir`). When the configured directory is non-empty but fails the stat/`IsDir` check, the function returns the home directory with a nil error and nothing is logged.

## Desired outcome

When a non-empty configured working directory is rejected and resolution falls back to home, the agent emits a diagnostic (a warn-level log with the configured and resolved paths) at the call sites that have a logger. The shared resolver stays logger-free.

## Constraints

- Keep the shared `usershell.ResolveWorkingDir` free of a logger dependency. The diagnostic belongs at the call sites.
- The SSH command path resolves against a per-session EnvInfoer (container override), so the log there should reflect the actual resolved directory rather than assume the host.

Follow-up to coder/coder#26102 (deep-review CRF-1).

> 🤖 Filed by Coder Agents on behalf of @mafredri.