fix(flow-ai): trim description under 1024-char install limit, add CI guard

[mhusbynflow]

What

• Fix the install blocker: the plugin installer rejects a SKILL.md whose frontmatter description exceeds 1024 characters. flow-ai's had grown to 1121, causing install errors. Trimmed to 988 by removing only redundant phrasing — every trigger keyword is preserved, so activation behaviour is unchanged.
• Add a lean CI guard: scripts/validate_skills.py (stdlib-only, no deps) checks that every plugins/**/SKILL.md has name + description and that description ≤ 1024 chars. Wired via .github/workflows/validate-skills.yml on PRs and pushes to main.
• Version bump 0.7.0 → 0.7.1 across plugin.json and all flow-ai/<version> User-Agent references, per the CHANGELOG's bump-both-together rule. CHANGELOG entry added.

Verification

• Validator is green on the current repo (Checked 1 skill(s), 0 error(s)., exit 0).
• Validator is red on an injected 1200-char description (description is 1200 chars (max 1024), exit 1).

Out of scope

• Making the workflow a required status check — that's a branch-protection ruleset setting and requires the workflow to have run on main first.
• Broader skill quality/best-practice linting (e.g. tessl scenario evals) — deliberately kept to the bare-minimum guard for now.

🤖 Generated with Claude Code

[mhusbynflow]

@claude please review this PR