Server(fix[sessions,clients]): Return empty on tmux errors

[tony]

Summary

• Restore lenient-by-default behavior for Server.sessions and Server.clients: tmux failures (socket permission errors, subprocess crashes, missing daemon, malformed output) once again yield an empty QueryList, matching the contract Server.sessions has carried since 0.17.0.
• Revert a change in 0.57.0 that made these accessors raise LibTmuxException. Code written for 0.57.0 with try/except LibTmuxException around these accessors keeps working — the except clause just becomes unreachable.
• Leave Server.search_sessions, Server.search_windows, and Server.search_panes raising — they take a caller-supplied filter, where a tmux error carries meaningful signal.
• Codify the convention in AGENTS.md so future list-returning accessors don't drift toward loud-failure by default.

Behavior

Before (0.57.0):

try:
    clients = server.clients
except LibTmuxException as exc:
    log(f"list-clients failed: {exc}")
else:
    if not clients:
        log("no clients attached")

After (this PR):

if not server.clients:
    log("no clients attached or tmux unreachable")

For an explicit connectivity signal, callers use Server.is_alive() or Server.raise_if_dead() — both available since 0.17.0.

Scope

Accessor	Behavior on tmux error
Server.sessions	empty QueryList (restored)
Server.clients	empty QueryList (restored)
Server.search_sessions	raises LibTmuxException (unchanged from 0.57.0)
Server.search_windows	raises LibTmuxException (unchanged from 0.57.0)
Server.search_panes	raises LibTmuxException (unchanged from 0.57.0)

Test plan

• test_server_sessions_returns_empty_on_tmux_error — Server.sessions returns QueryList([]) on simulated LibTmuxException
• test_server_clients_returns_empty_on_tmux_error — same for Server.clients
• test_server_sessions_permission_error_returns_empty — connection errors absorbed into the empty contract
• test_server_sessions_missing_socket_returns_empty — fresh Server(socket_path=...) against a non-existent path returns empty (unchanged)
• test_server_search_sessions_propagates_errors — search_* methods still raise (unchanged)
• uv run ruff check .
• uv run mypy
• uv run pytest --reruns 0
• just build-docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.37%. Comparing base (4acee52) to head (874108f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
+ Coverage   51.26%   51.37%   +0.11%     
==========================================
  Files          25       25              
  Lines        3482     3488       +6     
  Branches      686      686              
==========================================
+ Hits         1785     1792       +7     
+ Misses       1403     1401       -2     
- Partials      294      295       +1     

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