fix review findings: harden the app bins + built-bin smokes, arch-exception doc, snapshot fixture-guard

BLOCKER — the published lib/bin.js (stdio + acp) was exercised only via tsx
(demo:* / the src/bin.ts smokes); the built artifact under plain `node` was
unguarded. Root-cause on the BUILT bin:
  1. Settle race: boot() returned once loader.create() registered the include
     ENTRY, but the include loads its child plugins asynchronously — so boot()
     (and main()) resolved while the app plugins (stdin reader, agent loop, ACP
     bridge) were still mounting. A CLI with no attached handles yet exits 0
     silently, and a load error surfaces as an unhandled rejection AFTER boot.
     Fix: `await ctx.loader.await()` after create() — settle the whole tree.
  2. Config-path robustness: hand the include the config's ABSOLUTE file:// URL
     so resolution never depends on ctx.baseUrl / can never fall back to cwd.
Both bins fixed identically. NOTE: the cordis Loader resolves a config's bare
plugin specifiers via its internal module loader, active only under
`node --expose-internals`; the bin cannot add a node flag itself, so this is
documented in the bin JSDoc + both package READMEs (the demos already comply).
The repo `examples/*/cordis.yml` are tsx-only artifacts (workspace plugins
resolve through the tsconfig paths map, not node_modules), so they are not a
valid plain-node bin target — the smokes use a real-install-shaped temp dir.

Fail loud on a load failure: boot() previously exited 0 SILENTLY when a config
path's directory does not exist — the include plugin fails to IMPORT, the cordis
Loader catches+LOGS it and leaves the entry with no fiber (no rejection), and
`loader.await()` does not rethrow (EntryTree.await uses Promise.allSettled). Fix:
boot() now calls assertEntriesLoaded(ctx) after the tree settles and throws on
any entry with no fiber, so a typo'd config dir exits non-zero with a clear
message. main() also installs an unhandledRejection guard (installFailLoud) that
replaces Node's stack dump with a single labelled stderr line for the
companion case (a missing config FILE in a real dir, whose include-init throw
surfaces as a rejection Node already exits non-zero on). Regression tests added
to both built-bin smokes (missing dir + missing file → non-zero exit + stderr);
verified the missing-dir test fails on the pre-fix bin.

Built-bin smokes (the reviewer's ask): packages/ui/{stdio,acp}-agent/tests/
built-bin.e2e.ts run the REAL lib/bin.js under `node` (NOT tsx) in a temp
consumer dir, asserting the stdio echo round-trip / the acp initialize response
+ stdout purity, plus the fail-loud cases above. They build-gate (skip if lib/
absent) and run in a new ci.yml step after the build.

Issue 2 — packages/README.md + docs/architecture.md said "plugins depend on
interfaces, never on the concrete loop", but dsh-agent-core imports the concrete
dsh-agent-loop. Scope the rule to EXTENSION plugins and carve out the sanctioned
COMPOSITION/bundle exception (dsh-agent-core composes the concrete spine); note
it in the implemented RFC too.

Issue 3 — examples/acp-agent/tests/acp.snapshot.ts fixture-guard claimed
no-model scenarios need no session.jsonl, but runScenario() always boots
llm-replay with the session.jsonl path and loadReplayScript() throws when it is
absent. Require session.jsonl for ALL scenarios (no-model ones ship a
header-only fixture) and rewrite the comment to match reality.

docs/architecture.md

@@ -39,7 +39,7 @@ For a catalog of the **data structures** this architecture moves around — the
 └─────────────────────────────────────────────────────────────┘
 ```
 
-Dependency rule: plugins depend on interface packages, never on `dsh-agent-loop`. The loop itself is swappable — UI/hook/tool plugins keep working against the `dsh-agent` vocabulary if the loop is replaced.
+Dependency rule: **extension** plugins depend on interface packages, never on `dsh-agent-loop`. The loop itself is swappable — UI/hook/tool plugins keep working against the `dsh-agent` vocabulary if the loop is replaced. The one sanctioned exception is a **composition/bundle** package whose job IS to assemble the concrete spine: `dsh-agent-core` bundles `dsh-agent-loop` (and the other concrete spine plugins) by design, so it depends on the concrete loop on purpose. The rule constrains plugins that EXTEND the system, not the bundle that COMPOSES it — swapping the loop means publishing a different bundle, not rewiring every extension.
 
 ## Service map