bug(commands): /cost and /budget overlap, /exit and /quit redundant, /help hides Pi help

[akougkas]

Problem

Cost/budget overlap:

• /cost (dispatch domain): shows per-run cost breakdown from dispatch ledger
• /budget (scheduling domain): shows ceiling/spent/remaining from budget tracker
  Both display cost information from different data sources. Users cannot know which to use.

Exit redundancy:

• /exit (PanCode): calls ctx.shutdown()
• /quit (Pi): hidden by shell-overrides, dead
  Users typing /quit (common reflex) get nothing. No autocomplete suggestion.

Help replacement:

• /help (PanCode): custom categorized list excluding Pi commands
• Pi's built-in help: completely inaccessible
  Users cannot access Pi's native help or discover Pi-specific features.

Audit Reference

AUDIT-ISSUES.md: C5, C6, C7

Objective

Consolidate overlapping commands and improve discoverability.

Deliverables

• Merge /cost into /budget as a sub-command or section
• Or: remove /cost and enhance /budget with per-run breakdown
• Unhide /quit or register it as an alias for /exit
• Add "Pi SDK Commands" section to /help showing available Pi commands