feat(ssh): discover computers from ~/.ssh/known_hosts + remote system-prompt

Two improvements to the SSH support feature:

1. KNOWN_HOSTS DISCOVERY (packages/ssh):
   Computers are now auto-discovered from ~/.ssh/known_hosts (every hostname
   you've ever connected to) in ADDITION to ~/.ssh/config (explicit Host
   aliases). Config entries take precedence (full params); known_hosts entries
   get defaulted params (User=defaultUser, IdentityFile=null→pool probes
   default keys, Port from [host]:port or 22, knownHost=true). Zero-config —
   no ~/.ssh/config file needed; hosts just appear.

   Reject list: dispatch.toml [ssh].reject = [...] (glob patterns like
   github.com, *.ts.net) filters noise from the catalog. Read from both
   the global ~/.config/dispatch/dispatch.toml and the project dispatch.toml.
   Parsed with Bun.TOML.parse (zero deps). Only filters discovery (catalog);
   specific lookups (getComputer/getStatus/test/connect) ignore the reject
   list (it's a visibility filter, not access control).

   New pure functions: parseKnownHosts(), isRejected(), globMatch().
   +26 tests. tsc EXIT 0, biome clean, 1756 tests pass.

2. REMOTE SYSTEM-PROMPT AWARENESS (packages/system-prompt):
   When a conversation has a computerId set (remote turn), the system prompt
   now resolves system:os, system:hostname, git:branch/git:status, and
   file: reads against the REMOTE machine — not the local host. Previously
   the prompt always said 'Arch Linux (WSL)' + local hostname even when the
   agent was connected to a remote Artix Linux machine.

   The ResolverAdapters' hostname()/platform() are now async (so a remote
   adapter can run 'hostname'/'uname -s' over SSH). The system-prompt
   extension builds remote adapters from the ExecBackend (readFile→SFTP,
   spawn→SSH exec). Cache invalidation now checks computerId (switching
   computers rebuilds the prompt). The compaction path also threads
   computerId. @dispatch/system-prompt now depends on @dispatch/exec-backend.

1 files changed, 12 insertions, 6 deletions

diff --git a/packages/system-prompt/src/resolver.ts b/packages/system-prompt/src/resolver.ts
index f271f47..e864554 100644
--- a/packages/system-prompt/src/resolver.ts
+++ b/packages/system-prompt/src/resolver.ts
@@ -45,10 +45,16 @@ export interface ResolverAdapters {
 	readonly fs: ResolverFs;
 	/** Override the current time (defaults to `new Date()`). */
 	readonly now?: () => Date;
-	/** Override `process.platform` (defaults to the real platform). */
-	readonly platform?: () => string;
-	/** Override the hostname (defaults to `os.hostname()`). */
-	readonly hostname?: () => string;
+	/**
+	 * Override `process.platform` (defaults to the real platform). Async so a
+	 * remote adapter can run `uname -s` over SSH.
+	 */
+	readonly platform?: () => Promise<string>;
+	/**
+	 * Override the hostname (defaults to `os.hostname()`). Async so a remote
+	 * adapter can run `hostname` over SSH.
+	 */
+	readonly hostname?: () => Promise<string>;
 }
 
 /** Per-construction context forwarded by the session-orchestrator. */
@@ -157,9 +163,9 @@ export async function resolveVariables(
 	// ── system:* ────────────────────────────────────────────────────────────
 	vars.set("system:time", now.toISOString());
 	vars.set("system:date", now.toISOString().slice(0, 10));
-	const platform = adapters.platform?.() ?? process.platform;
+	const platform = (await adapters.platform?.()) ?? process.platform;
 	vars.set("system:os", await resolveOs(platform, adapters.fs));
-	vars.set("system:hostname", adapters.hostname?.() ?? osHostname());
+	vars.set("system:hostname", (await adapters.hostname?.()) ?? osHostname());
 
 	// ── prompt:* ────────────────────────────────────────────────────────────
 	vars.set("prompt:cwd", cwd);