Implement porthole v0.1: profiles, supervised open/close, reconnect

Full CLI per spec v0.2 - add/open/close/edit/status/list/remove/wipe/
completions, plus a hidden `__supervise` subcommand that IS the
supervisor process.

- profile.rs: TOML-backed profiles at ~/.config/porthole/profiles/,
  validated -l/-r/-d mapping + --via grammar, atomic writes.
- instance.rs: JSON runtime state at ~/.local/state/porthole/, an
  flock-based lock file that's the source of truth for "is this open"
  (survives a crash/kill -9 without stale-lock cleanup), pid liveness
  checked against /proc rather than trusted from disk.
- supervisor.rs: the __supervise loop - spawns ssh, traps SIGTERM/SIGINT
  into a flag (rather than inferring intent from ssh's exit status),
  classifies failures as fatal/known-transient/unrecognized, backs off
  with a stability-reset, rotates its log.
- ssh.rs: builds the ssh invocation, including splitting --via into a
  -J jump chain plus the mandatory positional target.
- open.rs: the detach/re-exec dance (setsid via pre_exec) and a bounded
  wait for the supervisor to reach Up/Error before open returns, so an
  immediate failure surfaces as a non-zero exit instead of a false
  "opened" - this took a real bug fix during smoke testing, since the
  instance file's initial state (Reconnecting, meaning "attempt in
  flight") was indistinguishable from "already failed once" by state
  alone.
- close.rs: SIGTERM+wait, or SIGKILL the whole process group with
  --force so ssh can't be left orphaned.

Smoke-tested against invalid/unreachable hosts (no real infrastructure
touched): CLI surface, validation errors, add/edit/list/status/remove,
the reconnect/backoff loop with live state transitions, close mid-retry,
edit-while-running's warning, open --all, and wipe. cargo test: 14/14.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/error.rs

@@ -0,0 +1,63 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum PortholeError {
+    #[error(
+        "invalid name '{0}' (use 1-64 chars: letters, digits, '_' or '-'; \
+         must start with a letter or digit)"
+    )]
+    InvalidName(String),
+
+    #[error("no profile named '{0}'")]
+    NotFound(String),
+
+    #[error("profile '{0}' already exists (use 'edit' to modify it)")]
+    AlreadyExists(String),
+
+    #[error("exactly one of -l/--local, -r/--remote, -d/--dynamic is required")]
+    NoMappingKind,
+
+    #[error("only one of -l/--local, -r/--remote, -d/--dynamic may be given")]
+    MultipleMappingKinds,
+
+    #[error("invalid forward spec '{0}': expected [bind:]port:host:hostport (or [bind:]port for -d)")]
+    InvalidMapping(String),
+
+    #[error("invalid --via hop '{0}': expected [user@]host[:port]")]
+    InvalidVia(String),
+
+    #[error("--via is required: at least one hop (its last entry is the ssh connection target)")]
+    NoViaHosts,
+
+    #[error("nothing to do: {0}")]
+    NothingToDo(String),
+
+    #[error("'{0}' failed to start: {1}")]
+    OpenFailed(String, String),
+
+    #[error(transparent)]
+    Io(#[from] std::io::Error),
+
+    #[error("state file error: {0}")]
+    Serde(String),
+}
+
+pub type Result<T> = std::result::Result<T, PortholeError>;
+
+impl From<toml::de::Error> for PortholeError {
+    fn from(e: toml::de::Error) -> Self {
+        PortholeError::Serde(e.to_string())
+    }
+}
+
+impl From<toml::ser::Error> for PortholeError {
+    fn from(e: toml::ser::Error) -> Self {
+        PortholeError::Serde(e.to_string())
+    }
+}
+
+impl From<serde_json::Error> for PortholeError {
+    fn from(e: serde_json::Error) -> Self {
+        PortholeError::Serde(e.to_string())
+    }
+}