test: cover the two locations the composition test was carrying

Moving `web-agent-presets` to the built lane took its coverage with it, and the
per-file gate named what it had been the only reach for: the session header's
`agentPreset` type check, and the preset subtree's `write()` override. Neither
belonged to a real-composition boot — a header validation case and a subtree
behaviour are package-level facts.

The `write()` test states why the override exists: the inherited method
persists the tree whenever the Loader thinks the config moved, and disposing an
agent disposes its subtree, so inheriting it truncates the shipped preset to
`[]` the first time a session ends.

packages/core/session/tests/session.spec.ts

@@ -1319,6 +1319,7 @@ describe('SessionStore', () => {
       { meta: { delegationDepth: '1' }, error: /delegationDepth must be a non-negative safe integer/ },
       { meta: { delegationDepth: 0.5 }, error: /delegationDepth must be a non-negative safe integer/ },
       { meta: { delegationDepth: -1 }, error: /delegationDepth must be a non-negative safe integer/ },
+      { meta: { agentPreset: 1 }, error: /agentPreset must be a string/ },
     ]
 
     for (const [index, { meta, error }] of cases.entries()) {

packages/preset/agent-presets/tests/mount.spec.ts

@@ -1,3 +1,4 @@
+import { readFile } from 'node:fs/promises'
 import { dirname, join } from 'node:path'
 import { fileURLToPath, pathToFileURL } from 'node:url'
 import { Context } from 'cordis'
@@ -217,6 +218,25 @@ describe('a roster with nothing in it', () => {
 })
 
 describe('attributing a service to a subtree', () => {
+  it('never writes the preset file back, however the subtree changes', async () => {
+    const handle = await ctx.agents.create({
+      sessionId: SessionId('sess-write'),
+      setup: async (agentCtx: Context) => void await ctx.agentPresets.mount(agentCtx, 'standard'),
+    })
+    const [mount] = livePresetMounts().filter(entry => entry.presetId === 'standard')
+    expect(mount).toBeDefined()
+    const file = join(FIXTURES, 'system', 'standard', 'agent.cordis.yml')
+    const before = await readFile(file, 'utf8')
+
+    // The inherited `write()` persists the tree whenever the Loader thinks the
+    // config moved, and disposing an agent disposes its whole subtree — which
+    // is enough to trigger it. Inheriting that truncates the shipped preset to
+    // `[]` the first time a session ends.
+    await handle.dispose()
+
+    expect(await readFile(file, 'utf8')).toBe(before)
+  })
+
   it('attributes nothing to a subtree that is already torn down', async () => {
     const handle = await ctx.agents.create({
       sessionId: SessionId('sess-torn'),