fix: detect mounts via /proc/self/mountinfo so stale FUSE works

mountinfo.Mounted lstats the path. When the sshfs link dies, every stat
on the mountpoint returns EIO, so -l filtered the dead mount out, mount
failed in MkdirAll, and -u failed before fusermount. Switch detection to
mountinfo.GetMounts (no stat) and add a fusermount -uz fallback so a
stale mount can actually be torn down.

DECISIONS.md

@@ -1,5 +1,45 @@
 # Decisions
 
+## 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
+  by `lstat`-ing the path. When a sshfs connection dies, the FUSE endpoint
+  stays in `/proc/mounts` but every `stat` on the mountpoint returns EIO.
+  All three sshfsc paths broke as a result:
+  - `-l` silently filtered the dead mount out (`Mounted` returned an error,
+    code skipped on `merr != nil`)
+  - mount path failed with `mkdir … file exists` because `MkdirAll` stats
+    before creating
+  - `-u` failed with `lstat … input/output error` before reaching
+    `fusermount`
+  Net effect: a stale mount was a dead end — couldn't be listed, couldn't be
+  unmounted, blocked re-mount.
+- **Tradeoffs**:
+  - Switched mount detection to `mountinfo.GetMounts(SingleEntryFilter(path))`,
+    which parses `/proc/self/mountinfo` and never touches the path. Robust to
+    EIO on the mount target.
+  - `list_mounts` now uses `PrefixFilter(base)` and trims the base prefix.
+    Direct children only — nested mounts under an alias dir would be skipped
+    on purpose; sshfsc only manages one level.
+  - `verify_mount_dir` skips `MkdirAll` when the path is already a mountpoint.
+    Re-running `sshfsc <alias>` against a stale mount no longer errors on
+    mkdir; main's existing `is_mounted_at` check then prints "Already mounted"
+    and returns. The user can `sshfsc -u <alias>` to recover.
+  - `unmount_sshfs` falls back to `fusermount -uz` (lazy) when `-u` fails.
+    Lazy unmount detaches the FS from the tree immediately and lets dangling
+    refs settle later — exactly what stale FUSE needs. Plain `-u` first
+    keeps the clean path noise-free.
+- **How**:
+  - New helper `is_mounted_at(path)`; replaces `mountinfo.Mounted` at every
+    call site (`main`, `verify_mount_dir`, `unmount_sshfs`).
+  - `list_mounts` rewritten around `PrefixFilter`. Trims `base + "/"` and
+    skips entries containing further separators.
+  - `run_fusermount(flag, mount)` extracted to keep `-u` and `-uz` calls
+    tidy. `unmount_sshfs` retries with `-uz` on first-call error.
+  - New tests: `TestIsMountedAtFalseOnPlainDir`,
+    `TestIsMountedAtFalseOnMissingPath`. Existing tests stay green —
+    `mountinfo.GetMounts` returns empty under the test tmpdirs.
+
 ## 2026-05-04 – Add `-l` (list) and `-u` (unmount) flags
 - **Who**: Dom, ClaudeCode
 - **Why**: Mounts under `$XDG_RUNTIME_DIR/sshfs/<alias>` had to be listed and