chore(agents): resolve contributor guidance review gaps

[purp]

User Story

I use OpenShell's contributor workflows to review and prepare repository changes. While reviewing PR #3987, I encountered incomplete label discovery and conflicting contribution instructions. I need the follow-up guidance to work consistently for contributors and maintainer agents.

Problem Statement

PR #3987 simplifies contributor guidance, but its bare gh label list commands omit lifecycle labels beyond the default 30 results. Triage no longer explicitly replaces its previous assessment state. Branch naming and verification instructions still disagree across skills and contributor guides, CI.md links to a removed vouch section, and automated dependency PRs have no documented exception to the issue requirement.

Impact / Why This Matters

Agents can miss workflow definitions, retain contradictory assessment states, or follow different branch and verification rules depending on the skill they load. Maintainers currently resolve these inconsistencies manually during review; repeated chat corrections do not fix the repository guidance. Automated dependency PRs also need an explicit issue-linkage exception.

Description

Prepare a focused draft follow-up based on PR #3987. Centralize branch naming in CONTRIBUTING.md, complete label discovery, preserve assessment-state replacement, align verification pointers, repair the vouch link, and document the automated dependency PR exception without changing OpenShell runtime behavior.

Context

• Depends on PR chore(agents): simplify contributor instructions and workflows #3987 and its originating issue chore(agents): refactor contributor instructions and workflows #3980.
• The user directly requested this follow-up after reviewing the specific findings.
• The PR should target chore(agents): simplify contributor instructions and workflows #3987's branch so reviewers see only these additional fixes.

Acceptance Criteria

• Label discovery retrieves the complete current label set and handles missing or ambiguous descriptions without guessing.
• Triage replaces the prior assessment state while preserving maintainer disposition and unrelated labels.
• CONTRIBUTING.md defines <type>/<issue>-<description>/<GitHub-username> as the branch naming convention; AGENTS.md, skills, and docs examples agree with it.
• Documentation and TUI instructions follow scoped verification guidance.
• Automated dependency PRs are explicitly exempt from separate issues and closing references; human-authored dependency updates retain the normal requirement.
• CI.md links to the retained vouch instructions.
• Changed guidance passes applicable formatting and consistency checks.