[FEATURE] User-designated pinned files with extended cache duration

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Current 5-minute prompt cache expires too quickly for iterative development workflows, causing:

• Repeated re-tokenization of frequently accessed files
• Increased token costs for users
• Unnecessary CPU/bandwidth usage for Anthropic's infrastructure

Benefits
For Users:

Lower token costs on frequently modified files
Faster response times (no re-processing)
Better workflow for iterative development
For Anthropic:

Reduced CPU usage (less re-tokenization)
Lower bandwidth (files stay in memory)
Predictable infrastructure costs

Use Case
Working on a SwiftUI app where ContentView.swift is modified 10+ times in 30 minutes. Currently requires re-caching 6+ times. With pinning: cache once, use throughout session. after 30 minutes recach is needed

Proposed Solution

Allow users to designate 2-3 "pinned" files with extended cache (15-30 minutes):

Option 1: Via .claude/claude.md

## Pinned Files
- src/ContentView.swift
- src/Model.swift

**Option 1:  claude --pin file1.swift --pin file2.swift

### Alternative Solutions

_No response_

### Priority

Critical - Blocking my work

### Feature Category

CLI commands and flags

### Use Case Example

_No response_

### Additional Context

_No response_