docs: add npx ECOMPROMISED recovery steps

[stablegenius49]

Summary\n- add a troubleshooting entry for npm ECOMPROMISED / Lock compromised failures during npx installs\n- document safe recovery steps: cache clean/verify first, then integrity-index-only reset if needed\n- include retry commands for both ruflo and claude-flow packages\n\n## Validation\n- git diff --check\n- attempted markdown lint, but README has many pre-existing lint violations unrelated to this change\n\nCloses #1231

[xkonjin]

Quick review pass:

• This looks docs-only from the diff, so I don’t see a code-path blocker here.
• Please still verify the examples/commands in README.md against current behavior so we don’t merge stale instructions.
• If there’s a linked feature change behind this, CI should include at least one smoke check that matches the documented flow.

[ruvnet]

Closing — the ECOMPROMISED error is an npm cache issue that's better documented in npm's own troubleshooting. Adding npm-specific recovery steps to our README would go stale quickly. Users can find the fix via npm cache clean --force. Thanks for flagging this!