refactor: prune superseded releases silently

[jiminu]

What

selfishell update no longer prints Removed inactive Selfishell release: X.
when it prunes a superseded release directory.

Before, on every CLI update:

Update Selfishell CLI to 1.2.15? [y/N] y
Removed inactive Selfishell release: 1.2.13.
Synchronize developer profile packages and configuration? [y/N] y

Selfishell updated: 1.2.14 -> 1.2.15

After:

Update Selfishell CLI to 1.2.15? [y/N] y
Synchronize developer profile packages and configuration? [y/N] y

Selfishell updated: 1.2.14 -> 1.2.15

Why

Retention keeps exactly the active release and the rollback release, so each
update supersedes one directory and printed one line, permanently.

• It restates a fixed contract rather than reporting news: docs/UPDATES.md
  already documents that only the active and rollback releases are retained.
• The directory is managed product state under
  ~/.local/share/selfishell/releases, not user data. Unlike a package install
  or a managed configuration change, it alters nothing the user owns or has to
  act on.
• The one actionable fact it carried -- which release can still be rolled back
  to -- is available on demand and more reliably from selfishell status
  ([CLI] Current: X | Rollback: Y), and selfishell rollback on a pruned
  version already fails with Retained release not found: X.
• The line count was not even fixed: a stale or interrupted update can leave
  several directories behind and print one line for each.

This follows the same rule as dropping update's N items unchanged. summary in
#187: report work that changed the user's environment, warnings, errors, and
required action; do not narrate an invariant.

Scope

One printf removed from release_prune_inactive, plus a comment recording
why the removal is silent. release_prune_inactive is called only from
release_install, so rollback output is unaffected. No test asserted the
string, and per AGENTS.md an incidental presentation change of this kind does
not warrant a new regression test.

Note the output lands before the exec continuation, so the line still appears
on the update into the release that contains this change and disappears from
the one after it -- the same rollout lag as the removed
Selfishell CLI updated to ... line in #187.

bash scripts/check.sh passes locally (345 tests, 0 failures).