Support session reuse across stacked PRs #103

When working with stacked PRs, pilot may spawn separate sessions for each PR in the stack even though they share context. A session reviewing PR #2 in a stack already has context about PR #1 below it.

Desired behavior:

• When pilot picks up a PR that's part of a stack, reuse an existing session that's already working on another PR in the same stack
• This avoids redundant context-gathering and gives the agent a better understanding of the full change

Stack detection: PRs where headRefName of one equals baseRefName of another, or via git-spice metadata.