feat: add --dir flag to /run, /chain, /parallel commands

[vedang]

Summary

Adds optional --dir <path> flag to /run, /chain, and /parallel slash commands to control working directories and isolate artifacts in task-specific folders.

Changes

• index.ts:
  • Added extractTrailingFlags() helper to parse --dir and --bg flags with quote-aware reverse-scanning
  • Added stripSurroundingQuotes() to handle quoted path values
  • Added takeTrailingArg() for robust argument extraction
  • Added applyInlineConfigToStep(), buildChainStep(), buildParallelTask() helpers to reduce duplication
  • Updated /run handler to set cwd when --dir provided
  • Updated /chain handler to inject chainDir when --dir provided
  • Updated /parallel handler to inject chainDir when --dir provided
  • Updated command descriptions to include --dir flag
• path-handling.test.ts: Added 11 comprehensive tests for slash command parsing
• README.md: Added documentation and examples for --dir usage
• Added investigation document session-persistence-investigation.md for reference

Behavior

• /run ... --dir <path>: Sets cwd parameter for the subagent run
• /chain ... --dir <path>: Injects chainDir for chain artifacts
• /parallel ... --dir <path>: Injects chainDir for parallel artifacts
• Relative paths resolved against ctx.cwd
• When --dir omitted, existing default behavior preserved
• Quoted task text containing --dir or --bg substrings correctly handled

Examples

# Isolate outputs in task-specific directory
/run scout "analyze auth system" --dir 20260304T145245--auth-audit/

# Chain with artifacts in workspace directory
/chain scout "collect context" -> planner "draft plan" --dir task-123/

# Parallel runs with shared directory
/parallel scout "scan frontend" -> reviewer "check security" --dir code-review/

Related Issue

Closes: #45