Fix what review found in the skill: five real defects

Adversarial review of #29 found bugs I confirmed by reproducing each one.

**Every hand-written error message was unreachable.** `tok=$(curl ...)` is a
plain assignment, so `set -e` acts on the command substitution before the
following `|| die` can run. A wrong password produced exit 22 and no output at
all — the most likely way this gets used wrongly, and the least explained.
All four captures now go through a `run` helper that takes a *description*
rather than echoing the command, because one of them carries the account
password in `-u`.

**`up` was not idempotent, and the second run destroyed DNS.** The resolv.conf
backup was copied unconditionally, so `up --full` twice overwrote the good
backup with PIA's own resolvers; the later `down` then "restored" those and
left the container with no working DNS and no way back. `up` now runs `down`
first. Verified: two `up --full` runs, then `down`, and the backup still holds
the original 192.168.65.7.

**An empty gateway produced total connectivity loss, reported as healthy.**
`$gw` was never validated and `add_route` swallowed every failure to /dev/null.
The two half-routes need no gateway and would succeed, so the tunnel captured
everything while the exclusions keeping DNS and the Docker host reachable
silently did not exist — and `status` still printed "full tunnel". Routes are
now fatal on failure, and a via-less default (`$3` is the literal "eth0") is
rejected.

**The PIA session token was in the process arguments** — confirmed in `ps` and
/proc/*/cmdline, a ~24h bearer credential for the account readable by anything
in the container. It now goes to curl on stdin as a config. Verified: 60 polls
across a full `up`, zero sightings.

**The preflight diagnosed the wrong kernel module.** It checked /dev/net/tun
and blamed the tun module, but kernel WireGuard is a netlink interface and does
not use it — verified by creating one with NET_ADMIN and no tun device. The
check is dropped (the container could not have started without the device
anyway) and `ip link add` now reports the real dependency.

Also: a full tunnel with no DNS servers from PIA used to warn and carry on,
which is a tunnel leaking every lookup while reporting itself healthy — now
fatal. `down` validates the backup before restoring it, so a truncated one
cannot leave the container with no resolver at all. `wg.priv` is shredded on
teardown and created under umask 077, because /run rides `docker commit` into
the snapshot image. A mistyped `up --ful` is rejected instead of silently
giving a test route.

entrypoint: `install_feature_skill` gets `local`, a blank-name guard (the
disabled branch would otherwise `rm -rf` the whole skills directory under a
persisted volume), `-e`/`-L` so a leftover *file* at the destination is cleaned
up, and a chown of the parent so `claude` can still add skills of their own
when Mission Control is off. When the base image predates the skill it now says
so instead of returning silently — and `/opt/triple-c-skills` joins
FEATURE_PROBES so the migration pre-flight reports it. Docs corrected to match:
neither half reaches an existing project without a migration.

`vpn_env_var` extracted and tested, pinning the property the whole removal path
rests on — that the variable is emitted as 0 rather than omitted.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src-tauri/src/docker/container.rs

@@ -841,6 +841,23 @@ type VpnHostConfigParts = (
 /// host-kernel module auto-loading. It is also enough to flush netfilter rules
 /// inside the container, so pair it with `sandbox_mode_enabled` advisedly.
 /// Hence opt-in, per project, rather than on for everyone.
+/// The env var `entrypoint.sh` installs and removes the `pia-vpn` skill from.
+///
+/// **Emitted either way, never omitted.** `~/.claude` is a persisted volume, so
+/// turning the toggle off has to actively tell entrypoint to remove a skill an
+/// earlier run left there, and an absent variable cannot say that. It is also
+/// what stops a `=1` baked into a snapshot by `docker commit` from outliving
+/// the setting — the explicit `=0` overwrites it.
+///
+/// Extracted for the same reason as [`vpn_host_config`]: the emitting code sits
+/// in a very long function where a dropped or inverted value is invisible, and
+/// `MISSION_CONTROL_ENABLED` twenty lines above shows the failure this avoids —
+/// it is pushed only when true, so a snapshot's baked `=1` survives the toggle
+/// going off.
+fn vpn_env_var(enabled: bool) -> String {
+    format!("VPN_SUPPORT_ENABLED={}", u8::from(enabled))
+}
+
 fn vpn_host_config(enabled: bool) -> VpnHostConfigParts {
     if !enabled {
         return (None, None, None);
@@ -1276,14 +1293,7 @@ pub async fn create_container(
         env_vars.push("MISSION_CONTROL_ENABLED=1".to_string());
     }
 
-    // Drives the pia-vpn skill install in entrypoint.sh. Sent as 0 rather than
-    // omitted when off, because ~/.claude is a persisted volume: entrypoint has
-    // to be told to *remove* a skill left there by an earlier run with the
-    // toggle on, and an absent variable cannot say that.
-    env_vars.push(format!(
-        "VPN_SUPPORT_ENABLED={}",
-        u8::from(project.vpn_support_enabled)
-    ));
+    env_vars.push(vpn_env_var(project.vpn_support_enabled));
 
     // Permission mode — read by triple-c-task-runner for scheduled (headless)
     // Claude Code runs. Interactive terminals get the flags directly instead.
@@ -2799,6 +2809,17 @@ mod tests {
         assert_eq!(cap_add.unwrap(), vec!["NET_ADMIN"]);
     }
 
+    #[test]
+    fn the_vpn_skill_flag_is_emitted_either_way_never_omitted() {
+        // The whole removal path depends on this. If `false` ever became "emit
+        // nothing", a project that had the toggle on would keep the skill
+        // forever: the container recreates from a snapshot whose baked
+        // VPN_SUPPORT_ENABLED=1 would then go unchallenged, and entrypoint
+        // would reinstall a skill for a capability the container no longer has.
+        assert_eq!(vpn_env_var(true), "VPN_SUPPORT_ENABLED=1");
+        assert_eq!(vpn_env_var(false), "VPN_SUPPORT_ENABLED=0");
+    }
+
     #[test]
     fn the_vpn_skill_flag_is_reserved_from_custom_env() {
         // entrypoint.sh installs and removes the pia-vpn skill from this