feat(mode): the session-mode core — logged per-agent policy state (@deepseek-ai/dsh-mode)

Plan mode's stage 1 (RFC 2026-07-07-plan-mode): a new packages/mode/ group
with one product package owning the mode/set SessionEventMap vocabulary
(log-only, non-surface, whole-value replace), the pure foldMode, and the
ctx.modes service (list/get/set). User flips are pending intents flushed
at turn/start / step/end — turn enclosure makes an idle append illegal —
with one coalesced context/message notice when the flushed mode differs
from what the last logged request header told the model; a folded mode
the config no longer defines reads as default plus one boundary notice.

Enforcement is two covering layers: a system-prompt/assemble wrapper
filters the RETURNED assembly's tools to the mode's allowlist (and shows
exit_plan_mode IFF the folded mode is plan) beside the mode:policy
section at order 50, and a tools/pre-execute gate denies deny-by-default
against the same allowlist, judging by the logged mode only. The default
mode is the absence of policy — assemblies stay byte-identical to a
no-dsh-mode deployment.

AgentOptions.mode (declaration-merged) seeds a child's initial mode
through the same flush on agent/created; the stdio app gains /mode
(print/switch, never sent to the model) over an opportunistic
ctx.get('modes'). Config is an explicit resolve step: the built-in plan
definition (read-only allowlist; bash/subagent excluded until the
sandbox family lands) merges unless overridden, 'default' as a key
throws at load, unknown names throw at set() time.

packages/ui/stdio-agent/package.json

@@ -37,6 +37,7 @@
     "@deepseek-ai/dsh-agent": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
     "@deepseek-ai/dsh-agent-core": "^0.0.1",
+    "@deepseek-ai/dsh-mode": "^0.0.1",
     "@deepseek-ai/dsh-session": "^0.0.1",
     "@deepseek-ai/dsh-tools": "^0.0.1",
     "@deepseek-ai/dsh-session-persistence-jsonl": "^0.0.1",
@@ -53,6 +54,7 @@
     "@deepseek-ai/dsh-agent": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",
     "@deepseek-ai/dsh-agent-core": "workspace:^",
+    "@deepseek-ai/dsh-mode": "workspace:^",
     "@deepseek-ai/dsh-system-prompt": "workspace:^",
     "@deepseek-ai/dsh-session": "workspace:^",
     "@deepseek-ai/dsh-tools": "workspace:^",