feat: warn and suggest install for opt-in launch commands

[harshitha-cstk]

What

Adds an init hook that intercepts commands belonging to demoted (now opt-in) plugins when the plugin isn't installed. For launch:*, instead of the generic "command not found", it prints:

Warning: "launch" is now an opt-in plugin and is not installed, so "launch:*" commands are unavailable.

Install it to enable these commands:
  csdx plugins:install @contentstack/cli-launch

Why

After the launch demotion (DX-9742), a bare "command not found" reads like a regression to users for whom launch:* previously worked out of the box. This gives an actionable hint pointing at the install command, consistent with the README opt-in note and the migration guide (DX-9748).

How

• New src/hooks/init/opt-in-plugin-guide.ts — runs on init (before command resolution), so it pre-empts @oclif/plugin-not-found cleanly. command_not_found hooks run concurrently and can't reliably suppress one another, so init is the correct interception point.
• Driven by a DEMOTED_PLUGINS map (launch → @contentstack/cli-launch) for easy extension.
• No-op when the plugin is installed (launch runs normally); genuinely unknown commands still fall through to plugin-not-found's "did you mean?".
• Registered under oclif.hooks.init in package.json.

Verification

node bin/dev.js launch / launch:<cmd> → prints the warning + install hint, exits 127. tsc --noEmit clean. Control command (foobar) unchanged.

Related: DX-9742 (demotion), DX-9748 (migration notes).

🤖 Generated with Claude Code

[github-actions]

🔒 Security Scan Results

ℹ️ Note: Only vulnerabilities with available fixes (upgrades or patches) are counted toward thresholds.

Check Type	Count (with fixes)	Without fixes	Threshold	Result
🔴 Critical Severity	0	0	10	✅ Passed
🟠 High Severity	1	37	25	✅ Passed
🟡 Medium Severity	0	2	500	✅ Passed
🔵 Low Severity	0	0	1000	✅ Passed

⏱️ SLA Breach Summary

✅ No SLA breaches detected. All vulnerabilities are within acceptable time thresholds.

Severity	Breaches (with fixes)	Breaches (no fixes)	SLA Threshold (with/no fixes)	Status
🔴 Critical	0	0	15 / 30 days	✅ Passed
🟠 High	0	0	30 / 120 days	✅ Passed
🟡 Medium	0	0	90 / 365 days	✅ Passed
🔵 Low	0	0	180 / 365 days	✅ Passed

ℹ️ Vulnerabilities Without Available Fixes (Informational Only)

The following vulnerabilities were detected but do not have fixes available (no upgrade or patch). These are excluded from failure thresholds:

• Critical without fixes: 0
• High without fixes: 37
• Medium without fixes: 2
• Low without fixes: 0

✅ BUILD PASSED - All security checks passed