feat: share clients — t=type manual mountpoint (existing path without create)

User report: smb-client/nfs-client mountpoint step could only auto-suggest
candidates, or create a fresh dir behind a hidden 'n=new' key — no way to
type an arbitrary existing path as the mountpoint, so the manual option
was effectively invisible (candidates from /media etc. always populated
the picker, hiding the typing path entirely). Designer framing: capability
gap + discoverability gap; backend already handled arbitrary paths (CLI
cmd_mount + ensure_mountpoint), only the interactive menu blocked it.

Change (identical in bin/pos-share-smb-client and bin/pos-share-nfs-client):
- pick_mountpoint hint 'n=new' -> 't=type'; key arm n -> t
- ask_new_mountpoint generalized to ask_mountpoint: an existing
  directory is now used AS-IS (no create, no confirm); a non-existent
  path keeps the 'Create mountpoint?' confirm + sudo mkdir flow; existing
  non-directory rejected ('has a file there'); shape checks and system-path
  refusal unchanged; stream contract (display->stderr, path->stdout) kept
- menu_ask_mountpoint empty-candidate fall-through now routes through the
  same ask_mountpoint validator (single source of truth)

Docs: DOC/howto/share.md NFS+SMB mountpoint sections updated from n=new to
t=type and describe existing-path-without-create behavior.

Scoped to the two client files + howto doc; persistence/automount units,
unmount/remove flows, cmd_* CLIs, share_folder_candidates, and
lib/menu-lib.sh untouched.

Verified: 9-scenario smoke matrix x2 files (~19 assertions each: existing
dir as-is, new-dir confirm+create, decline, relative/trailing-slash/system/
empty rejections, non-dir reject, mkdir-fail), make gen idempotent, make
check OK, make lint 0 FAIL/0 WARN, make test 17 files / 299 checks green,
bash -n clean, git diff --check clean. Designer ACCEPT framing+spec;
Reviewer ACCEPT after doc fix.

DOC/AGENT_Context_Project.md

@@ -644,9 +644,9 @@ Use conventional prefixes: `feat:`, `fix:`, `docs:`, `refactor:`, `chore:`
 | `bin/pos-network-hotspot` | 93 | Wi-Fi hotspot via create_ap + wihotspot-gui |
 | `bin/pos-network-ip` | 69 | Show interfaces, routes, public IP + location |
 | `bin/pos-network-scan` | 272 | Parallel ping sweep of CIDR |
-| `bin/pos-share-nfs-client` | 504 | Mount NFS shares (ephemeral or persistent systemd mount units) |
+| `bin/pos-share-nfs-client` | 511 | Mount NFS shares (ephemeral or persistent systemd mount units) |
 | `bin/pos-share-nfs-server` | 245 | Manage the NFS kernel server (status, share/unshare exports, enable/disable) |
-| `bin/pos-share-smb-client` | 766 | Mount SMB/CIFS shares (ephemeral or persistent systemd mount units) |
+| `bin/pos-share-smb-client` | 773 | Mount SMB/CIFS shares (ephemeral or persistent systemd mount units) |
 | `bin/pos-share-smb-server` | 441 | Manage the Samba server (status, share/unshare exports, users, enable/disable) |
 | `bin/pos-share-usb-server` | 362 | USB Redirector server control (--ls, --share; prompts when args omitted) |
 | `bin/pos-ssh-load-keys` | 31 | Load all SSH keys into the agent |