feat(add): support targeted agent installs

[knoxgraeme]

Summary

Non-interactive installs can now target one or more detected agents instead of always installing to every detected agent. Repeat --agent <name> to select multiple agents; matching is case-insensitive, duplicates are ignored, and any unknown request stops the install before files are written. Omitting the option preserves the current interactive picker and all-detected non-interactive default.

Validation

• npm run typecheck
• npm run lint
• npm run format:check
• npm test (249 tests)
• npm run build
• CLI help smoke: npx tsx src/index.ts add --help

Post-Deploy Monitoring & Validation

• Search user reports and CLI output for Agent not found after release.
• Confirm targeted installs list only the requested agent names and JSON failures return exit code 4.
• Healthy signal: no increase in failed skillfish add runs and no reports of installs reaching unrequested agents.
• Failure trigger: any targeted install writes to an unrequested agent, or valid detected names are rejected; mitigate by reverting this commit and restoring the all-detected selection path.
• Validation window and owner: maintainer, first release cycle after publish.

---