Document the codebase thoroughly and tighten type safety

Docs: per-folder README.md for packages/ (family overview + one per
package: service, events, API, extension points, TODOs), examples/,
and examples/echo-agent/; folder-level AGENTS.md (+ CLAUDE.md
symlinks) for packages/ and vendor/; module-level doc comments in
every packages/*/src file; richer JSDoc on all exported API
(event side effects, disposal contracts, error behavior). Root
AGENTS.md gains a "Type Safety and Documentation" policy section:
the codebase aims to be very type-safe and well documented; type
gymnastics are acceptable in core packages when they improve
plugin-author DX; verbose docs are fine as long as they stay strictly
in sync with the code.

Type safety: removed the upstream-inherited "noImplicitAny": false
from tsconfig.base.json — packages/* now compile under full strict
mode; vendor/loader and vendor/include set it locally (vendor/cordis
already did). Eliminated every `: any` / `as any` from packages and
examples (catch clauses use unknown + a CodedError narrowing type;
event data access uses discriminated-union narrowing).

Typed tool schemas: new @deepseek-ai/dsh-tools schema DSL —
SchemaSpec with per-property `required: true` booleans, type-level
InferArgs<S>, a runtime SchemaSpec → JSON Schema converter, and
defineTool() so first-party tools get typed execute(args) with zero
casts (raw JSON Schema still accepted for MCP interop; chosen over
schemastery because it targets JSON Schema generation directly).
echo-tool and all test tools migrated; +7 tests.

packages/session/src/index.ts

@@ -1,3 +1,11 @@
+/**
+ * Event-sourced session service: append-only session log, in-memory store, and
+ * the derived LLM message history. Persistence is a plugin concern (subscribe
+ * to `session/event`, drain on `session/flush`).
+ *
+ * @module @deepseek-ai/dsh-session
+ */
+
 import { Context, Service } from 'cordis'
 import type { ContentBlock, Message, MessageSource } from '@deepseek-ai/dsh-llm'
 import type { SessionEvent, SessionEventMap, SessionEventType } from './types.ts'
@@ -64,7 +72,11 @@ export class Session {
     return this.log.length
   }
 
-  /** Append one event. Synchronous — the hot path never blocks on I/O. */
+  /**
+   * Append one typed event to the log and synchronously notify observers via
+   * `onAppend`. The hot path never blocks on I/O — persistence plugins buffer
+   * asynchronously.
+   */
   append<T extends SessionEventType>(type: T, data: SessionEventMap[T]): SessionEvent<T> {
     const event = { type, seq: this.log.length, time: Date.now(), data } as SessionEvent<T>
     this.log.push(event)
@@ -132,7 +144,12 @@ export class SessionStore extends Service {
     super(ctx, 'sessions')
   }
 
-  /** Create a session. `seed` replays/forks an existing event log. */
+  /**
+   * Create a session. If `seed` is provided, the session is populated with
+   * a copy of those events (replay/fork). The session is a Cordis effect:
+   * disposing the calling fiber stops event notification and removes the
+   * session from the store.
+   */
   create(id?: string, seed?: SessionEvent[]): Session {
     id ??= `session-${++this.counter}`
     if (this.store.has(id)) throw new Error(`session "${id}" already exists`)