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/AGENTS.md

@@ -0,0 +1,27 @@
+# AGENTS.md — Harness Packages
+
+This directory contains all `@deepseek-ai/dsh-*` harness packages. When editing
+code here, follow these conventions:
+
+- **Effect-based registrations**: every contribution (tool, section, adapter,
+  agent, event listener) goes through `ctx.effect()` / `ctx.on()`, and
+  `register()` methods return disposers. Never use bare arrays or manual cleanup.
+- **Declaration merging**: services declare their ctx key in
+  `declare module 'cordis' { interface Context { } }` and their events in
+  `interface Events`. Merge-extensible maps (`ContentBlockMap`,
+  `MessageSourceMap`, `FinishReasonMap`, `TurnTriggerMap`, `TurnEndReasonMap`,
+  `SessionEventMap`) are how plugins add new variants.
+- **Waterfall semantics**: `ctx.waterfall` listeners receive `(...args, next)`;
+  call `next()` to delegate, or return without it to short-circuit (veto). Never
+  call `next()` after returning.
+- **Tests**: vitest in `packages/<name>/tests/*.spec.ts`. Every registry needs an
+  HMR-safety test (register a plugin, dispose its fiber, assert cleanup). Err on
+  the side of more tests — edge cases, error paths, event ordering, races.
+
+Naming notes:
+- Files `src/index.ts` export the service default + all public types
+- `src/types.ts` contain only types — no runtime code
+- Tests live at package level under `tests/`, not `src/__tests__/`
+
+Read the per-package README.md for package-specific details: service API,
+events, extension points, TODOs.