add README for vite_task_plan

Author: branchseer

crates/vite_task_plan/README.md

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+# vite_task_plan
+
+Execution planning layer for the vite-task monorepo task runner. This crate converts abstract task definitions from the task graph into concrete execution plans ready for execution.
+
+## Overview
+
+`vite_task_plan` sits between [`vite_task_graph`](../vite_task_graph) (which defines what tasks exist and their dependencies) and the actual task executor. It resolves all the runtime details needed to execute tasks:
+
+- Environment variables (fingerprinted and pass-through)
+- Working directories
+- Command parsing and expansion
+- Process spawn configuration
+- Caching metadata
+
+## Key Concepts
+
+### Execution Plan
+
+The main output of this crate is an [`ExecutionPlan`](src/lib.rs), which contains a graph of task executions with all runtime details resolved.
+
+```rust
+let plan = ExecutionPlan::plan(plan_request, cwd, envs, callbacks).await?;
+```
+
+### Plan Requests
+
+There are two types of execution requests:
+
+1. **Query Request** - Execute tasks from the task graph (e.g., `vite run -r build`)
+   - Queries the task graph based on task patterns
+   - Builds execution graph with dependency ordering
+
+2. **Synthetic Request** - Execute on-the-fly tasks not in the graph (e.g., `vite lint`)
+   - Generated dynamically with provided configuration
+   - Used for built-in commands
+
+### Execution Items
+
+Each task's command is parsed and split into execution items:
+
+- **Spawn Execution** - Spawns a child process
+  - Contains: resolved env vars, cwd, program/args or shell script
+  - Environment resolution for cache fingerprinting
+
+- **In-Process Execution** - Runs built-in commands in-process
+  - Optimizes simple commands like `echo`
+  - No process spawn overhead
+
+- **Expanded Execution** - Nested execution graph
+  - Commands like `vite run ...` expand into sub-graphs
+  - Enables composition of vite commands
+
+### Command Parsing
+
+Commands are intelligently parsed:
+
+```bash
+# Single command -> Single spawn execution
+"tsc --noEmit"
+
+# Multiple commands -> Multiple execution items
+"tsc --noEmit && vite run test && echo Done"
+#     ↓              ↓                ↓
+# SpawnExecution  Expanded      InProcess
+```
+
+### Error Handling
+
+- **Recursion Detection** - Prevents infinite task dependency loops
+- **Call Stack Tracking** - Maintains task call stack for error reporting