feat: install the stack, policy and secrets skills

[arunim2405]

Summary

• The installer now installs all six skills. SG_SKILL_NAMES (src/sg/skills.ts) listed only the three workflow skills. So sg-create-stack, sg-create-policy and sg-manage-secrets were in skills/ and shipped in the npm package and the binary, but were never installed. The three are now added to the list.
• Tests: the unit test that pins the list, the binary e2e test that compares each installed SKILL.md to its source, and the status e2e assertion (3 skills → 6 skills). The unit test's check for unprefixed skill references now also covers create-stack, create-policy and manage-secrets.
• skills/README.md is rewritten for people onboarding through an agent. It covers how the agent, skills and MCP server fit together, StackGuardian terms, a "which skill do I need" table, and for each skill: example requests, a collapsible step-by-step walkthrough, and "good to know" notes.
• README: "Check that it works" lists all six slash commands. The Skills table now has six rows, a "Try saying" column and a link to skills/README.md.
• 1.1.0 with a CHANGELOG entry. It's a minor bump because users get new skills.

The companion PR StackGuardian/skills#4 says add-sg-mcp installs all six skills. Merge that one after this is released.

Test plan

• bun run typecheck
• bun run test: 22/22 test files pass, including sg-binary (compiled executable installs all six embedded skills) and sg-cli (status reports 6 skills)
• bun run build
• Prettier clean on every changed file. The 6 synced skills/sg-*/SKILL.md files already fail prettier --check on main; this PR doesn't touch them.
• After release: npx add-sg-mcp@latest on a machine with 1.0.6 skills installed, then confirm /sg-create-stack, /sg-create-policy and /sg-manage-secrets appear in Claude Code
• Check skills/README.md rendering on GitHub (tables, collapsible sections, anchors)

🤖 Generated with Claude Code