feat: add hash-based file change detection (works without Git)

[vjache]

Description

Fixes #6 - Adds automatic reindexing via file watcher and hash-based change detection for VCS-agnostic operation.

When CIE runs as MCP server, it now:

1. Watches files for changes and auto-triggers incremental reindex
2. Uses hash-based detection (no Git required) by default
3. Logs all events to .cie/index.log for diagnostics
4. Provides cie_reindex tool for explicit reindexing from AI clients

Related Issue

Fixes #6

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Testing

• Tested locally with KiloCode MCP client
• File watcher detects changes and triggers reindex automatically
• Hash-based detection works without Git repository
• cie_reindex tool callable from MCP client

Key Changes

• pkg/ingestion/hash_delta.go - Hash-based change detection (VCS-agnostic)
• cmd/cie/watch.go - File watcher with debounce for auto-reindex
• pkg/ingestion/index_log.go - Structured logging to .cie/index.log
• cmd/cie/config.go - Watch: true, UseGit: false by default
• cmd/cie/mcp.go - cie_reindex tool and reindex lifecycle logging

Backwards Compatibility

Fully backwards compatible. Existing behavior available via config:

• indexing.use_git: true - Use git-based detection
• indexing.watch: false - Disable file watcher

Checklist

• My code follows the project's code style
• I have tested this locally
• My changes generate no new warnings