Gantry: 10 - Lifecycle-bound background work

## Scope

Bind best-effort background work to agent lifecycle.

## Problem

`firePrefetch` and `reAdvertiseDigest` start goroutines with `context.Background()`, so shutdown cannot cancel them and manifest bursts can create unbounded concurrent best-effort work.

Evidence:

- `internal/gantry/mirror/mirror.go:1756`
- `internal/gantry/mirror/mirror.go:1774`

## Changes

- Pass an agent-scoped context or a cancellable child into `firePrefetch` and `reAdvertiseDigest` instead of `context.Background()`.
- Add a small semaphore if manifest bursts create too many concurrent goroutines.
- Ensure shutdown cancellation stops these best-effort goroutines before discovery/libp2p teardown completes.

## Tests

- Lifecycle cancellation stops prefetch and readvertise goroutines.
- Shutdown does not start new background prefetch/readvertise work after drain begins.

Start in internal/gantry/mirror/mirror.go at lines 1756 and 1774, tracing firePrefetch and reAdvertiseDigest through the agent lifecycle and shutdown path. Run or add lifecycle tests covering cancellation and shutdown drain behavior; done means both goroutine types stop on cancellation and no new work starts after drain begins.

Written by the indexing model from the issue text.