refactor(cli/load,tests) Migrate display-message to typed wrapper

[tony]

Summary

• Drop the last two cmd("display-message", "-p", ...) escape hatches in the codebase: one in production (cli/load._reattach), six in test code (the plugin-system suite).
• Same tmux invocation under the hood, just routed through libtmux's typed Pane.display_message(cmd, get_text=True) wrapper. No behavior change.
• Follows the libtmux 0.56.0 bump in py(deps) Bump libtmux 0.56.0 #1038 — the wrapper itself has been around since 0.55.0, but with 0.56.0 now landed there's no remaining reason to keep the cmd() escape for this particular subcommand.

Why now

PR #1038 broadened the Pane.display_message flag coverage in libtmux 0.56.0 (format_string=, verbose=, delay=, notify=, target_client=). While auditing tmuxp for spots that could move to the new typed surface, the only remaining cmd() escape in production turned out to be the post-reattach session-name echo loop in cli/load._reattach. Five of the six other usages were in tests asserting on session/window names via the same idiom — leaving them on cmd() while production migrated would split the call shape across the codebase.

The migration itself doesn't depend on a 0.56.0-only API; this is consistency cleanup that 0.56.0's broader display_message surface made an obvious moment to do.

Changes in this PR

c54893e5 — cli/load(refactor[_reattach]) Use Pane.display_message wrapper

Production change. Replaces:

proc = builder.session.cmd("display-message", "-p", "'#S'")
for line in proc.stdout:
    ...

with:

active_pane = builder.session.active_pane
assert active_pane is not None
lines = active_pane.display_message("'#S'", get_text=True)
for line in lines:
    ...

The narrowing assert mirrors the existing assert builder.session is not None already at the top of _reattach(). tmux's display-message without -t defaults to the session's active pane, so the underlying invocation is byte-for-byte identical.

46b52f16 — tests(refactor[plugin_system]) Use Pane.display_message wrapper

Migrates six test sites in tests/cli/test_cli.py::test_reattach_plugins and four test_plugin_system_* checks in tests/workspace/test_builder.py to the same typed wrapper. Hoists active_pane once in the multi-query test that asserts on both '#S' and '#W' (replaces two separate cmd() roundtrips with two calls on the same pane reference).

Test plan

• uv run ruff check . --fix --show-fixes clean before each commit
• uv run ruff format . clean before each commit
• uv run mypy clean before each commit (the narrowing assert was load-bearing — without it, Pane | None typing rejects the call)
• uv run py.test --reruns 0 -vvv — 797 passed, 2 skipped, before each commit
• just build-docs succeeds before each commit
• No remaining cmd("display-message", ...) calls anywhere in src/ or tests/ (verified via grep -rn 'cmd("display-message"' src/ tests/)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.97%. Comparing base (c675892) to head (46b52f1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1039      +/-   ##
==========================================
+ Coverage   81.96%   81.97%   +0.01%     
==========================================
  Files          28       28              
  Lines        2545     2547       +2     
  Branches      485      485              
==========================================
+ Hits         2086     2088       +2     
  Misses        328      328              
  Partials      131      131              

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