feat(lsp): LSP capability seam, generic stdio provider, and lsp tool

Implements the LSP capability seam RFC as three packages: dsh-lsp (the
ctx.lsp interface — provider registry by branded id + exclusive extension
mapping, per-query order-independent selection, closed request/result
vocabulary, LspError taxonomy), dsh-lsp-local (a generic stdio language-server
provider — Content-Length JSON-RPC framing, per-(provider, workspace) process
single-flight, transient didOpen/query/didClose, an abortable per-instance
queue, UTF-16 negotiation, host-namespace source reads outside ctx.fs, and
bounded shutdown/kill teardown), and dsh-tool-lsp (the model-facing lsp tool —
four operations, one-based UTF-16 cursor conversion, workspace-grouped location
rendering, hover capping, a required session workspace, and a timeout budget).

Why: an agent had text search and file reads but no way to identify a program
symbol — follow an alias, connect an interface to implementations, or read an
inferred type — before changing code. Splitting model contract, seam, and local
subprocess behavior keeps the four semantic queries stable across future remote
or sandbox-native providers without leaking a JSON-RPC escape hatch.

packages/lsp/tool-lsp/src/session-cwd.ts

@@ -0,0 +1,19 @@
+/**
+ * Derive the workspace root an `lsp` call resolves against: the calling agent's per-session
+ * workspace (`exec.agent.session.header.cwd`), mirroring how the filesystem tools resolve paths.
+ * Unlike those tools, LSP has NO provider fallback — a missing cwd fails the call as
+ * `LSP_WORKSPACE_REQUIRED`, because the local provider must canonicalize a real workspace before it
+ * can start a server.
+ * @module @deepseek-ai/dsh-tool-lsp/session-cwd
+ */
+
+import type { ToolExecution } from '@deepseek-ai/dsh-tools'
+
+/**
+ * The session workspace cwd for this call, or `undefined` when none applies.
+ * @param exec - the tool-execution context; only its optional `agent` is read.
+ * @returns the calling agent's session cwd, or undefined for a non-agent caller.
+ */
+export function sessionCwd(exec: ToolExecution): string | undefined {
+  return exec.agent?.session.header.cwd
+}