feat: add streamd lsp subcommand with LSP server

Implements a full Language Server Protocol server accessible via
`streamd lsp`. The server communicates over stdin/stdout and auto-
detects the workspace root from the LSP initialize request.

Features implemented:
- Passive mode when no .streamd.toml exists in workspace root
- Workspace-root-based config loading (bypasses R22/R23 global config)
- .streamd.toml file watcher (config reloads without restart)
- textDocument/completion with @ trigger, conditional suggestions
  (if_with relationships), and temporal date/time snippets (R16)
- textDocument/publishDiagnostics: R15 file-name format + R18
  timesheet violations (overlapping timecards, unclosed days)
- textDocument/documentSymbol: shard tree exposed as nested symbols
- textDocument/codeAction: "Mark task as done" quickfix for @Task
- workspace/symbol: cross-file shard search
- textDocument/references: find all @Marker occurrences
- textDocument/rename: rename @Marker across all files

Dependencies added: tower-lsp 0.20, tokio, dashmap, serde_json

src/cli/args.rs

@@ -72,4 +72,7 @@ pub enum Commands {
         #[arg(value_enum)]
         shell: Shell,
     },
+
+    /// Start LSP server (communicates over stdin/stdout)
+    Lsp,
 }