refactor: report one update result

[jiminu]

What

selfishell update now closes with one result line for a successful version
change, instead of a lower-level CLI success line, an aggregate
N items unchanged. summary, and a separate tools/configuration line.

Normal version update:

Installing node@24.19.0...
Updated managed configuration: nvim

Selfishell updated: 1.2.10 -> 1.2.14

--cli-only:

Selfishell updated: 1.2.10 -> 1.2.14

Already current, and --tools-only:

Selfishell is up to date (1.2.14).
Selfishell tools and configuration synchronized.

Release details stay on the GitHub Release rather than being reproduced in the
CLI.

Implementation

• command_update owns the closing result; release_install no longer prints
  its own Selfishell CLI updated to ... line.
• The continuation reads the version it replaced from the previous-release
  link, which release_install rewrites immediately before switching
  current. Nothing has to survive its exec, so an update launched by a CLI
  that predates this change still reports the transition.
• --cli-only keeps running in the process that performed the switch and
  reports the transition from the version it read before switching.
• --tools-only closes with synchronized rather than updated: that phase
  resynchronizes whether or not anything changes, and with the unchanged
  summary gone its result is often the only output, so it must not imply a
  change. Synchronize is already this phase's own word in its confirmation
  prompt, its help text, and docs/UPDATES.md.
• The default N items unchanged. summary is gone from update.
  selfishell install keeps its own: an explicit one-shot setup reports
  idempotence usefully, a routine update does not.

Trade-off

The result closes the whole command, so declining the tools/configuration
confirmation ends the run without reporting the version change even though the
CLI release has already switched. docs/UPDATES.md records that and points at
selfishell version.

Tests

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

• --cli-only reports the transition exactly once and no longer prints the
  release-installation success line.
• A default update reports OLD -> NEW after continuing with the new CLI,
  which also proves the previous-release link carried the source version
  across the exec.
• --tools-only keeps its own result, prints no version transition, and no
  longer prints the unchanged-items summary.
• Every assertion on the tools/configuration result follows the new wording,
  including the three failure-path checks that assert it is absent; leaving
  them on the old string would have turned them into checks that always pass.
• The two release-failure tests that asserted no success line was printed now
  target the line that reports success, instead of a string that no longer
  exists.

Out of scope

No PR or issue summaries, CHANGES assets, changelog metadata, GitHub API
calls for update presentation, global --quiet/--verbose framework, output
aggregation abstractions, package-manager upgrade changes, or unrelated
update/rollback refactoring.