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/README.md

@@ -0,0 +1,65 @@
+# dsh-session
+
+Event-sourced session log and in-memory store. A `Session` is the append-only
+source of truth for an agent's whole interaction history — the LLM message
+history is *derived* from it.
+
+## Service: `SessionStore` (ctx key: `sessions`)
+
+Creates and holds event-sourced `Session` instances. Persistence is intentionally
+not implemented here — plugins subscribe to `session/event` and flush on
+`session/flush`.
+
+### Public API
+
+- `ctx.sessions.create(id?: string, seed?: SessionEvent[]): Session`
+  Create a session. `seed` replays/forks an existing event log. Disposed with
+  the calling fiber.
+- `ctx.sessions.get(id: string): Session | undefined`
+- `ctx.sessions.list(): Session[]`
+
+### Events
+
+| Event | Mode | Purpose |
+|---|---|---|
+| `session/created` | emit | A session was created |
+| `session/event` | emit | An event was appended (sync, fire-and-forget) |
+| `session/flush` | parallel | Awaited durability checkpoint (persistence plugins drain buffers here) |
+
+### Class: `Session`
+
+Plain class (not a Cordis Service). Create via `ctx.sessions.create()`.
+
+- `session.append(type, data): SessionEvent` — synchronous, never blocks on I/O.
+- `session.deriveMessages(): Message[]` — derive the LLM message history from
+  the event log. Raw `assistant/chunk` events are skipped; `context/message` and
+  `steering/message` render as tagged synthetic user messages.
+- `session.events`, `session.seq`, `session.id`
+
+### Session event vocabulary (`types.ts`)
+
+The append-only log: `turn/start`, `turn/end`, `step/start`, `step/end`,
+`user/message`, `assistant/message`, `assistant/chunk`, `tool/call`,
+`tool/result`, `steering/message`, `context/message`, `usage`, `error`.
+
+Merge-extensible via `SessionEventMap` — a compaction plugin adds
+`compaction/marker`, etc.
+
+Also defines `TurnTriggerMap` and `TurnEndReasonMap` (merge-extensible sum types
+for typed turn boundaries — `kind`-tagged instead of strings).
+
+### Extension points
+
+- Persistence plugins: subscribe to `session/event` (write-behind) and drain on
+  `session/flush` (awaited) and fiber dispose. See
+  `examples/echo-agent/src/session-jsonl.ts` for the pattern.
+- Replay/fork: `ctx.sessions.create(id, seed)` seeds a new session with an
+  existing event log.
+
+### What is NOT here (TODO)
+
+- **Real persistence backends** (JSONL per session dir, sqlite) — future phase.
+- **Session event vocabulary review** — `TODO(review)` once the loop and a
+  persistence plugin coexist.
+- **Session branching/tree** (pi-style entry tree) — defered unless needed beyond
+  seed-based forking.