feat: add issue tracker discovery step to /plan skill

Description

Currently the /plan skill transforms brainstorm output into an implementation plan by reviewing
the codebase, performing optional research, and running flow analysis. It does not check the
project's issue tracker for related context.

Proposal: Add an optional step early in the /plan flow that searches the issue tracker (starting
with GitHub Issues via gh) for issues related to the current task. This would surface:

• Related or duplicate issues with prior discussion
• Assigned issues that overlap with the planned work
• Blockers, dependencies, or acceptance criteria others have already captured

Suggested behavior:

• Extract keywords from the brainstorm doc or task description
• Run a search against the issue tracker
• Present a brief summary of relevant issues found
• Let the user select which issues to incorporate into planning context

Open questions for discussion:

• Should this be opt-in (flag/prompt) or on-by-default with a skip option?
• How do we handle projects that don't use GitHub Issues (Linear, Jira, etc.)? MCP servers could
  extend support later.
• How aggressive should keyword extraction be? Too broad = noise, too narrow = missed context.
• Should it also check open PRs for in-flight work that overlaps?

Requirements

No response

Additional Context

No response

Start by reading the /plan skill and tracing its current brainstorm, codebase review, optional research, and flow-analysis steps. Inspect how GitHub Issues are accessed through gh; done should be defined around an optional search, a brief result summary, and user-selected planning context, while resolving the listed open questions.