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.

knip.json

@@ -118,6 +118,11 @@
       "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts", "tests/fixture-server.ts"],
       "project": ["src/**/*.ts", "tests/**/*.ts"],
       "ignoreDependencies": ["@modelcontextprotocol/server-everything", "@modelcontextprotocol/server-filesystem"]
+    },
+    "packages/lsp/lsp-local": {
+      "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts", "tests/fixture-server.ts"],
+      "project": ["src/**/*.ts", "tests/**/*.ts"],
+      "ignoreDependencies": ["typescript-language-server"]
     }
   }
 }