Implement v0.3.0 MCP security hardening (#91)

[Dimwiddle]

Description

Implement the remaining v0.3.0 security scope from #91.

What changed

• Add strict Click-level input validation callbacks for IDs/text and apply them across features, status, next, and enforce
• Harden SKILL file sync behavior so modified .specleft/SKILL.md is not regenerated unless explicit force is provided
• Add specleft init --force for intentional SKILL regeneration
• Surface skill_file_regenerated in init outputs (CLI JSON and MCP specleft_init)
• Expand MCP contract payload guarantees with MCP Security and Risk management #91 security assertions (cli_rejects_shell_metacharacters, init_refuses_symlinks, no_network_access, no_telemetry, etc.)
• Update generated SKILL safety guidance and CLI reference docs
• Add/update command, MCP, and acceptance tests for these paths

Type of Change

• Security hardening
• Feature enhancement
• Tests
• Documentation

Testing

• make lint
• pytest -q

Related Issues

Closes #91