fix(opencode): use commands/ directory (plural) to match OpenCode docs (#2453)

* fix(opencode): use commands/ directory (plural) to match OpenCode docs

OpenCode documentation (https://opencode.ai/docs/commands/) uses
.opencode/commands/ (plural) as the canonical command directory.
The OpenCode runtime supports both .opencode/command/ and
.opencode/commands/ via a {command,commands} glob, but the
singular form was the original convention and is now outdated.

Update the OpenCode integration to write to .opencode/commands/
instead of .opencode/command/, aligning with the documented
standard and the OpenSpec fix (Fission-AI/OpenSpec#748).

Signed-off-by: Marcus Burghardt <maburgha@redhat.com>
Assisted-by: OpenCode (claude-opus-4-6)

* feat(registrar): add legacy_dir fallback for backward-compatible directory migration

Add _resolve_agent_dir() to CommandRegistrar that checks a
legacy_dir fallback when the canonical directory does not exist.
When legacy_dir is found, a deprecation warning directs users to
run "specify integration upgrade" to migrate.

The OpenCode integration declares legacy_dir: ".opencode/command"
so that extension and preset registration, as well as command
cleanup, continue working for projects that have not yet migrated
to .opencode/commands/.

The legacy_dir mechanism is opt-in: integrations that do not
declare it get no fallback and no behavioral change.

Add end-to-end test verifying that "specify integration upgrade
opencode" migrates commands from legacy .opencode/command/ to
canonical .opencode/commands/ and removes stale files.

Signed-off-by: Marcus Burghardt <maburgha@redhat.com>
Assisted-by: OpenCode (claude-opus-4-6)

* fix(registrar): address PR review feedback on legacy_dir handling

- Fix deprecation warning formatting: quote paths and remove trailing
  '/.' that produced confusing '.opencode/commands/.' output
- Eliminate duplicate warnings: pass pre-resolved directory to
  register_commands() via _resolved_dir parameter so
  _resolve_agent_dir() is only called once per agent
- Fix unregister_commands() to clean both canonical and legacy dirs
  when both exist, preventing orphaned command files after upgrade
- Add test_unregister_cleans_legacy_when_both_dirs_exist regression
  test and tighten warning count assertion to exactly 1

Assisted-by: OpenCode (claude-opus-4-6)
Signed-off-by: Marcus Burghardt <maburgha@redhat.com>

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Signed-off-by: Marcus Burghardt <maburgha@redhat.com>