refactor: delegate -l output to findmnt

The custom one-alias-per-line output was useless — no mountpoint, no
source, no options. Reinventing a table format when findmnt from
util-linux already produces a familiar fuse.sshfs view was the wrong
call. -l now shells out to findmnt -t fuse.sshfs.

DECISIONS.md

@@ -1,5 +1,35 @@
 # Decisions
 
+## 2026-05-04 – Delegate `-l` output to `findmnt -t fuse.sshfs`
+- **Who**: Dom, ClaudeCode
+- **Why**: The custom `-l` output ("just the alias, one per line") was
+  uselessly minimal — no mountpoint, no source, no options. Reinventing a
+  table format for a single command was the wrong call when `findmnt` from
+  `util-linux` already produces a familiar, well-formatted view of fuse.sshfs
+  mounts.
+- **Tradeoffs**:
+  - External dependency on `findmnt` (part of `util-linux`, present on every
+    Arch and most other Linux systems by default — no real adoption cost).
+  - Output is mountpoint-first, not alias-first. The alias is only visible as
+    the last path segment under the SOURCE column / TARGET basename.
+  - Lists *all* fuse.sshfs mounts on the system, not just sshfsc-managed
+    ones under `$XDG_RUNTIME_DIR/sshfs/`. In practice this is what the user
+    wants ("show me all sshfs mounts"); a future `-r` (restrict) flag could
+    narrow it if needed.
+  - `findmnt` exits 1 when nothing matches its filter — we treat that as
+    success-with-empty-output, not a failure.
+- **How**:
+  - `list_mounts` rewritten as a thin wrapper around `exec.Command("findmnt",
+    "-t", "fuse.sshfs")` with stdout piped to the caller's writer.
+  - Exit-code 1 from `findmnt` is swallowed via `errors.As(err, *exec.ExitError)`
+    + `ExitCode() == 1`.
+  - `errors.Is(err, exec.ErrNotFound)` produces a clear "install util-linux"
+    message instead of a cryptic exec error.
+  - Tests `TestListMountsMissingBase` and `TestListMountsFiltersUnmounted`
+    removed: they covered the old in-Go enumeration logic that no longer
+    exists. Mocking `findmnt` would be alibi-testing — the wrapper is two
+    branches over `cmd.Run()`'s error.
+
 ## 2026-05-04 – Detect mounts via `/proc/self/mountinfo` instead of `stat`
 - **Who**: Dom, ClaudeCode
 - **Why**: `mountinfo.Mounted(path)` from `github.com/moby/sys/mountinfo` works