bug: Accept underscore modules in PR titles

[idy]

Background

The shared PR-readiness validator currently accepts only lowercase letters,
digits, and hyphens inside each title-prefix segment. Consumer repositories use
stable snake_case module and board identifiers such as zero_esp, so a title
like h106/zero_esp: add the Zero ESP Main App package is rejected even though
it accurately names the owning module and follows the repository naming guide.

Goal

• Allow _ inside non-empty lowercase PR title-prefix segments while preserving
  the existing prefix: Subject structure and lowercase first-character rule.
• Keep malformed titles, empty hierarchy segments, spaces, uppercase prefixes,
  and missing subjects rejected.
• Document the accepted prefix characters and cover the consumer regression.

Non-goals

• Do not change Issue Type, closing-Issue, PR body, review-model, or merge
  eligibility behavior.
• Do not relax prefix casing, hierarchy, colon-spacing, or subject requirements.
• Do not update consumer workflow pins as part of this repository change.

Code Changes Tree

.github/
└── scripts/
    ├── issue-review/
    │   └── common.mjs             # allow underscores inside lowercase title-prefix segments
    └── pr-readiness/
        └── test.mjs               # cover snake_case module acceptance and malformed-prefix rejection
README.md                          # document the exact accepted prefix characters and hierarchy form

Design

Extend the shared PREFIXED_TITLE expression so every slash-separated prefix
segment still begins with a lowercase ASCII letter and may then contain
lowercase ASCII letters, digits, hyphens, or underscores. Preserve the existing
single : delimiter and non-empty subject requirement.

The PR-readiness validator continues to import this shared expression. Add a
regression case for h106/zero_esp: add the Zero ESP Main App package plus
negative cases that demonstrate uppercase, empty segments, spaces, leading
underscores, and missing subjects remain invalid.

Test And Acceptance Criteria

Acceptance Criteria

• h106/zero_esp: add the Zero ESP Main App package has no invalid-title
  blocker.
• Existing lowercase and hyphenated prefixes remain accepted.
• Uppercase, space-containing, empty, or underscore-leading segments and empty
  subjects remain rejected.
• README wording matches the deterministic validator.

Validation

node .github/scripts/issue-review/test.mjs
node .github/scripts/pr-readiness/test.mjs
node .github/scripts/pr-review/test.mjs