feat(simplify): track file hashes to prevent repeated simplification proposals

[kostyay]

• Add SHA-256 hashing of file content to detect when files have actually changed since last simplification
• Persist simplified file hashes to $PI_CODING_AGENT_DIR/simplify-hashes.json, keyed by project directory to avoid collisions across workspaces
• Skip simplification proposals for files whose content matches their stored hash, eliminating redundant proposals after a simplify run completes
• Replace hasProposedSimplify flag with pendingSimplifyFiles to properly distinguish between agent-initiated simplification turns and user-initiated ones
• Update hash store after each simplification run completes, ensuring only genuinely modified files are proposed for future simplification
• Update changelog and extension documentation to reflect the new hash-based deduplication behavior