fix(ci): exclude intentional README template placeholders

[ss-o]

Summary

• exclude only the five intentional unresolved links in the Zsh plugin README
  template
• keep the template itself and its real documentation, logo, wiki, and
  organization links under Lychee checks
• trigger the Lychee workflow when .github/lychee.toml changes

Closes #548
Unblocks #546 and #547 after this change reaches main.

Verification

• pinned Lychee v0.18.1 against templates/readme/zsh-plugin.md:
  9 Total, 4 OK, 5 Excluded, 0 Errors
• actionlint .github/workflows/lychee.yml
• trunk check .github/lychee.toml .github/workflows/lychee.yml
• python3 -m unittest scripts/test_validate_agent_policy.py -v
  (83 tests passed)
• python3 scripts/validate-agent-policy.py
• git diff --check

Scope

The exclusion patterns match the encoded <repository> URL placeholder and
the two template-relative destinations LICENSE and
repository-owned-asset-path. The template file is not excluded wholesale.

Because the existing workflow uses pull_request_target with the default base
checkout, its PR run will continue to observe the current failing main
configuration. The push run after merge is the authoritative confirmation that
the baseline is repaired.