Revisit "dir breaks Coder Desktop file sync" warning after Mutagen fix ships

## Summary

Once the Coder Desktop file sync fix ships and is adopted, the `coder_agent.dir` warning about breaking file sync is no longer accurate and should be revisited (likely removed), coordinated with the existing `dir` deprecation.

## Background

Setting `coder_agent.dir` (or a devcontainer `workspaceFolder`) to a path other than `$HOME` breaks Coder Desktop file sync. Root cause: Mutagen copies its agent binary with `scp` and executes it over `ssh` using a `~/`-prefixed path; when the remote SSH/SFTP working directory isn't `$HOME`, `scp` writes the binary relative to that directory while the `ssh` exec expands `~` to `$HOME`, so installation fails.

Tracking:
- coder/coder-desktop-macos#238 (`coder_agent.dir` variant)
- Linear DEVEX-522 (devcontainer `workspaceFolder` variant)

A fix is proposed in the Mutagen fork: **coder/mutagen#10** — resolve the absolute remote home directory once and use an absolute path for both the copy and the exec, so they agree regardless of the remote working directory. This addresses both the `dir` and devcontainer variants.

## Where the warning lives

- `provider/helpers/validation.go` — `WarnDirNotHome` (the plan/apply-time warning):
`Setting "dir" to a value other than $HOME will break Coder Desktop file sync.`
- `provider/agent.go` — the `dir` field `Description` links the same phrasing to the docs.
- `docs/resources/agent.md` — generated from the schema description.

## Why not now

- coder/mutagen#10 is still a draft; nothing is released. The fix must land, be released, and be bundled into the desktop clients (`.mutagenversion` bump in both macOS and Windows) before the warning is stale.
- **Version skew:** this warning fires at template plan/apply time and cannot know which Coder Desktop / Mutagen version end users run. Removing it before broad adoption would regress users on older clients whose file sync is still broken.
- `dir` is **separately deprecated** (IDEs / `display_apps` reasons). That deprecation stands regardless of the file-sync fix, so the warning/field will likely be retired via the deprecation cleanup rather than reverted here.

## Proposed action (revisit when ready)

1. Confirm coder/mutagen#10 is merged, released, and bundled into the macOS and Windows Coder Desktop clients, and that both the `dir` and devcontainer variants are verified fixed.
2. Decide a min-version / adoption-window gate before changing user-facing guidance.
3. Update or remove the "breaks Coder Desktop file sync" wording in `WarnDirNotHome` and the `agent.go` `dir` description (regenerate `docs/resources/agent.md`), coordinated with the `dir` deprecation plan.
4. Update related docs references (e.g. `coder/coder` `docs/user-guides/desktop/desktop-connect-sync.md`).

## Acceptance

- The "breaks Coder Desktop file sync" claim is no longer surfaced once the fix is broadly available, without regressing users on pre-fix clients.

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_{🤖 Opened by Coder Agents on behalf of @aqandrew.}

Start by checking coder/mutagen#10 and the macOS and Windows client release status, then read WarnDirNotHome in provider/helpers/validation.go and the dir description in provider/agent.go. Review the generated docs/resources/agent.md and related desktop documentation. Done means the warning wording and generated docs are updated or removed only after broad fix adoption, with the dir deprecation coordinated and older clients protected.

Written by the indexing model from the issue text.