boot S5: two-identity share exemplar — prove a shared feed's arrival dirties the right listings

Part of epic #367. Plan: `~/.claude/plans/boot-cache.md`.

The case that motivated the whole epic is a colleague sharing a feed and the client staying silent about it. Job completion can be triggered with one identity and is covered by #367 S2's exemplar. Arrival cannot: someone has to share a feed with someone else.

## Work

A live exemplar using the existing `mise-e2e` identity as the second party. It shares a feed; a private test daemon running as the first identity observes the roster delta, the dirty mark on the affected listings, and the parent folder update.

The exemplar **skips when the second identity is absent**, exactly as the existing PACS and admin exemplars do, so it never turns CI red on a machine without both sets of credentials.

## Why it matters

A shared feed whose id is below the client's highest known feed id cannot be found by an id-ordered boot delta at all, so today it is invisible until a full roster walk happens to run. This is the only test that proves the design's central claim rather than asserting it against a fake.

## Test isolation

Never run against the user's daemon session. The private daemon must set all four isolations: `TMPDIR`, `XDG_RUNTIME_DIR`, `XDG_CONFIG_HOME`, `XDG_CACHE_HOME`.

Read ~/.claude/plans/boot-cache.md and the existing PACS and admin exemplars first, then follow the mise-e2e identity setup to locate the live test entry point. Use a private daemon with TMPDIR, XDG_RUNTIME_DIR, XDG_CONFIG_HOME, and XDG_CACHE_HOME isolated, and skip when the second identity is absent. Done means the exemplar observes the roster delta, affected-listing dirty marks, and parent-folder update.

Written by the indexing model from the issue text.