fix(completion): install static script instead of eval at login

[kallioli]

Context

orca completion install bash (and zsh) used to append
eval "$(_ORCA_COMPLETE=bash_source orca)" to the rc file. Each shell
startup re-spawned orca (which auto-walks ~60 command modules at
import time) plus a bash --version subprocess from Click's
_check_version(), costing 1–3 s per login.

A real incident on 2026-04-27 saw hundreds of orca processes pile
up on a single SSH session, all stuck inside _check_version()
waiting on their bash --version subprocess. The user-visible symptom
was a multi-thousand-line traceback after Ctrl-C.

Changes

• orca_cli/core/shell_completion.py — install_completion_bashzsh
  now generates the completion script once into
  $XDG_DATA_HOME/orca/completion.<shell> (XDG-spec compliant) and
  rewrites the rc to source that file. No orca process is spawned
  at login; sourcing a static script is microseconds.
• Migration: any pre-existing
  eval "$(_ORCA_COMPLETE=...)" line is detected and replaced on
  re-install, preserving surrounding rc content. Idempotent.
• orca_cli/commands/completion.py — INSTRUCTIONS rewritten for
  the manual two-step flow (generate static file + source).
• Lazy completion callbacks in orca_cli/core/completions.py are
  unchanged — tab completion still hits the API on demand with the
  per-profile cache and 5-minute TTL added in v2.0.1.
• docs/adr/0010-static-completion-script.md — full context,
  decision, alternatives considered, migration instructions.

Tests

TestInstallCompletionBashZsh rewritten and extended (5 tests):

• writes the static script and emits a source line
• appends alongside existing rc content
• idempotent re-install (no duplication)
• migrates legacy eval to source line
• fails gracefully when orca is not on PATH

pytest -q --cov=orca_cli --cov-fail-under=85: 2338 passed, coverage
88.42%. ruff + mypy clean.

Risks

• Behavioural change for existing users: the rc no longer contains
  the eval line. Re-running orca completion install bash migrates
  silently; users who never re-run install will keep the slow eval
  until they do. Mitigation: documented in CHANGELOG and ADR 0010.
• Two files installed instead of one rc line. Uninstall now needs
  to remove both. Documented in the install message.

Migration

orca completion install bash    # detects legacy eval and replaces it
exec \$SHELL                     # pick up the new sourced file