AIP-12: Add feature spec for Review PR: Reference Resolution & Guide

[jodavis-claude]

Work item: AIP-12 — Deliverable 1 of the Code Review Helper Detailed Design (parent epic AIP-4): resolves a PR reference to a single target PR, stands up an isolated worktree/clone, gathers evidence, and generates the standalone review guide.

Changes:

• Adds _spec_ReviewPrGuide.md, the full implementation-level dev spec for this deliverable.
• Defines the /dev-team:review-pr <ref> [prose] command shape, the six-component breakdown (resolve-pr-reference, create-review-worktree, gather-pr-evidence, write-review-guide, review-session-context, decide_cleanup_action.py), and their interfaces.
• Breaks the work into 6 tracked tasks (AIP-18 through AIP-23), 5 of them independent and parallelizable, with AIP-23 (wiring the command) depending on all 5.

Design decisions:

• Same-repo PRs use the native EnterWorktree tool; cross-repo PRs use a sibling-clone git worktree add when one exists, otherwise a scratch gh repo clone — EnterWorktree has no notion of an arbitrary other repo.
• resolve-pr-reference is a skill wrapping a fully pytest-testable script, plus exactly one inline MCP call for the Jira remote-links lookup (the one step no script in this repo can perform).
• Work-item-id resolution matches directly against configured issue-key-pattern/recognize-patterns rather than calling identify-project-work-items, since that skill's own ask-the-user fallback doesn't fit this fully-automated (and, for gather-pr-evidence, subagent) context.
• Cleanup is split into a pure decision script (decide_cleanup_action.py) and the actual tool/CLI action performed by review-pr, since ExitWorktree — like the decision logic itself — is only callable from an agent's own tool-use turn.

Testing completed: This PR adds the spec only (no implementation yet); the spec itself went through 3 rounds of automated readiness review (researcher-dev-spec-review) covering component-breakdown completeness, verification-mechanism feasibility, and cross-checks against this repo's actual tool/script capabilities.