Fix dynamic completions (regression)

[DannyBen]

cc #96

Summary

This PR fixes a regression introduced in v0.7.4 where dynamic completions like $(...) were collapsed into a single malformed completion token.

What Changed

• Fixed dynamic completion serialization in Pattern so dynamic $(...) entries are handled separately from literal words.
• Kept the v0.7.4 escaping behavior for literal completions with spaces/quotes.
• Added marker-based handling in the generated template filter to split dynamic command output into completion candidates.
• Centralized the dynamic marker value in Completely::Pattern::DYNAMIC_WORD_PREFIX and consumed it from ERB.

Tests and Validation

• Added integration coverage for mixed dynamic + literal completion behavior.
• Confirmed generated script remains:
  • shellcheck compliant
  • shfmt compliant
• Verified manual repro flow with completely test "cli ".

Docs / Developer Experience

• Added AGENTS.md guidance for this repo.
• Added :script_quality spec tag for shellcheck/shfmt checks.
• Updated spec/README.md to recommend respec and document quick commands.

[DannyBen]

This is available as a pre-release (0.8.0.rc1) on RubyGems:

gem install completely --pre