feat(server): add CLI upgrade command (#374)

## Summary
- Adds a `--upgrade [version]` CLI flag that upgrades the global
CodeNomad CLI server package and exits.
- Uses `bun add --global` for the package upgrade path and includes
server-side tests.
- Rebased onto the latest `dev` because we do not have permission to
push to the original fork branch.

## Credits
- Original PR: #363
- Original author: Pascal André (@pascalandr)

## Testing
- Not run; this PR only recreates the rebased branch from #363.

---------

Co-authored-by: Pascal André <pascalandr@gmail.com>

packages/server/src/cli-upgrade.test.ts

@@ -0,0 +1,39 @@
+import assert from "node:assert/strict"
+import { describe, it } from "node:test"
+import { buildUpgradeCommand, detectPackageManager, formatUpgradeCommand } from "./cli-upgrade"
+
+describe("cli upgrade", () => {
+  it("defaults to npm when no package manager can be detected", () => {
+    assert.equal(detectPackageManager({}), "npm")
+  })
+
+  it("detects package managers from npm user agent", () => {
+    assert.equal(detectPackageManager({ npm_config_user_agent: "pnpm/9.0.0 node/v22" }), "pnpm")
+    assert.equal(detectPackageManager({ npm_config_user_agent: "bun/1.0.0" }), "bun")
+    assert.equal(detectPackageManager({ npm_config_user_agent: "npm/10.0.0 node/v22" }), "npm")
+  })
+
+  it("builds latest upgrade command by default", () => {
+    const command = buildUpgradeCommand(undefined, "npm")
+
+    assert.equal(command.packageSpec, "@neuralnomads/codenomad@latest")
+    assert.deepEqual(command.args, ["install", "-g", "@neuralnomads/codenomad@latest"])
+    assert.equal(formatUpgradeCommand(command), "npm install -g @neuralnomads/codenomad@latest")
+  })
+
+  it("builds a versioned upgrade command", () => {
+    const command = buildUpgradeCommand("0.10.5", "pnpm")
+
+    assert.equal(command.packageSpec, "@neuralnomads/codenomad@0.10.5")
+    assert.deepEqual(command.args, ["install", "-g", "@neuralnomads/codenomad@0.10.5"])
+    assert.equal(formatUpgradeCommand(command), "pnpm install -g @neuralnomads/codenomad@0.10.5")
+  })
+
+  it("uses bun add for Bun installs", () => {
+    const command = buildUpgradeCommand("0.10.5", "bun")
+
+    assert.equal(command.packageSpec, "@neuralnomads/codenomad@0.10.5")
+    assert.deepEqual(command.args, ["add", "-g", "@neuralnomads/codenomad@0.10.5"])
+    assert.equal(formatUpgradeCommand(command), "bun add -g @neuralnomads/codenomad@0.10.5")
+  })
+})