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, 19 insertions, 4 deletions

diff --git a/packages/ssh/src/service.ts b/packages/ssh/src/service.ts
index 6a809c6..629c9f8 100644
--- a/packages/ssh/src/service.ts
+++ b/packages/ssh/src/service.ts
@@ -21,7 +21,7 @@ import type { ComputerStatusResponse, TestComputerResponse } from "@dispatch/tra
 import type { ComputerService } from "@dispatch/transport-http/dist/seam.js";
 import type { Computer, ComputerEntry } from "@dispatch/wire";
 import { createSshExecBackend } from "./backend.js";
-import { resolveComputer, resolveComputers } from "./config.js";
+import { resolveComputer, resolveComputers, type SshConfigResolveEnv } from "./config.js";
 import { createSshConnectionPool, type SshConnectionPool, type SshPoolDeps } from "./pool.js";
 
 /**
@@ -42,6 +42,12 @@ export interface SshServiceDeps extends SshPoolDeps {
 	 * host-bin wires this from conversation-store; absent → every count is 0.
 	 */
 	readonly getUsageCounts?: () => Promise<ReadonlyMap<string, number>>;
+	/**
+	 * Optional: glob patterns to exclude from the computer catalog (e.g.
+	 * `github.com`, `*.ts.net`). Sourced from `dispatch.toml` `[ssh].reject`.
+	 * Absent → no filtering.
+	 */
+	readonly readRejectPatterns?: () => Promise<readonly string[]>;
 }
 
 /** Build the `ComputerService` + the remote-`ExecBackend` factory. */
@@ -53,12 +59,21 @@ export function createSshService(deps: SshServiceDeps): {
 } {
 	const pool = createSshConnectionPool(deps);
 
-	async function readEnv() {
-		const [configText, knownHostsText] = await Promise.all([
+	async function readEnv(): Promise<SshConfigResolveEnv> {
+		const [configText, knownHostsText, rejectPatterns] = await Promise.all([
 			deps.readConfigText().catch(async () => ""),
 			deps.readFileText(deps.knownHostsPath).catch(async () => ""),
+			deps.readRejectPatterns !== undefined
+				? deps.readRejectPatterns().catch(async () => [] as readonly string[])
+				: Promise.resolve([] as readonly string[]),
 		]);
-		return { configText, knownHostsText, defaultUser: deps.defaultUser, homeDir: deps.homeDir };
+		const base: SshConfigResolveEnv = {
+			configText,
+			knownHostsText,
+			defaultUser: deps.defaultUser,
+			homeDir: deps.homeDir,
+		};
+		return rejectPatterns.length > 0 ? { ...base, rejectPatterns } : base;
 	}
 
 	const service: ComputerService = {