Keep a typed path as typed, and expand ~ where it is used

A llama session's tools all answered "failed to spawn process
[exit code: -1]": `llama-server` takes the working directory as an
`x-tool-cwd` header and `chdir`s to it with no shell in the way, so a
`~/…` cwd named a directory of that name. `files::resolve_blocking`
asks the machine that will serve the session what the path is, and the
driver does that once at launch.

The other half is the storing. `~` and `/home/someone` are a path and a
snapshot of where it pointed, and it is the snapshot that breaks when an
account is renamed -- so `machines::tidy` no longer expands one and
`set_cwd` no longer contracts one (`shorten_home` is gone with it). The
identity file is expanded at the point `ssh` is invoked instead.

Spawn now asks the same question of a typed working directory that
`set_cwd` already did: absolute or home-relative, and actually there on
the machine that will run it. It accepted anything, so a typo became a
session whose process could not start, reported later and pointing at
nothing.

Also: a provider written before `mcp_servers` existed adopts the
defaults a probe would give it now, so a machine discovered before
2026-09-19 stops silently having no web search.

Exercised end to end against a real llama session spawned with
`cwd: "~/repos/ai-app/server"`: `exec_shell_command` with `pwd` answered
`/home/bob/repos/ai-app/server`, exit 0.

AGENTS.md

@@ -471,6 +471,22 @@ written, and the fold uses that same predicate to decide a reply is settled.
   `generate` now fails the turn for both -- a reply that stops early is not a
   reply, and the transcript keeps whatever arrived before it.
 
+- **A path is stored as it was typed, and `~` is expanded where it is used.**
+  `~/repos/x` and `/home/someone/repos/x` are a path and a snapshot of where it
+  pointed, and the snapshot is what breaks when an account is renamed or the
+  value is read on another machine -- so nothing at the boundary rewrites one
+  in either direction (`machines::tidy` used to expand and `shorten_home` used
+  to contract; both are gone). Expansion belongs to the machine the path is on:
+  `ssh::quote_path` and `files::PATH_PRELUDE` for a remote one,
+  `ssh::expand_home` for one here. The exception that proves it is
+  **`llama-server`'s tools**, which take the working directory as an
+  `x-tool-cwd` header and `chdir` to it with no shell in the way: a `~` arrives
+  there as a directory of that name and *every* tool using one answers "failed
+  to spawn process\n[exit code: -1]", which on the phone looks like a session
+  whose tools are all broken. `files::resolve_blocking` is what the llama
+  driver resolves it with at launch, on the machine that will serve the
+  session.
+
 - **A transcript outlives the enum.** Removing `Event::TaskNote` hours after
   adding it made every transcript that had recorded one unreadable, so
   `launch` failed for those sessions and `SessionManager::new` skipped them —