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.

docs/cordis-catalog/services.md

@@ -147,6 +147,20 @@ Types: [GenerateOptions](../core-data-structures/core.md) · [StreamChunk](../co
 
 Source: [`packages/llm/llm/src/index.ts:88`](../../packages/llm/llm/src/index.ts)
 
+## `ctx.modes` — `ModesService`
+
+`ctx.modes`: the session-mode service. Owns the `mode/set` vocabulary, the pending-intent flush, the boundary narration, and both policy layers (the assemble filter + `mode:policy` section, and the `tools/pre-execute` gate). UIs read mode flips off `session/event`; there is no live mirror.
+
+```ts cordis-catalog
+list(): string[]
+get(agent: Agent): { current: string, pending?: string }
+set(agent: Agent, mode: string): void
+```
+
+Types: [Agent](../core-data-structures/core.md)
+
+Source: [`packages/mode/mode/src/index.ts:179`](../../packages/mode/mode/src/index.ts)
+
 ## `ctx.sessionPersistence` — `SessionPersistence` (abstract seam)
 
 Abstract durable session-persistence service. Subclass, implement the abstract methods, and load the subclass as a plugin — it registers as `ctx.sessionPersistence` (one implementation per context; loading a second throws, cordis' standard duplicate-service behavior).