fix(kiro-cli): replace literal $ARGUMENTS with prose fallback (#2482)

* fix(kiro-cli): replace literal $ARGUMENTS with prose fallback

Kiro CLI file-based prompts do not natively substitute any
argument placeholder (kirodotdev/Kiro#4141, kiro.dev/docs/cli
manage-prompts), so the literal "$ARGUMENTS" set in
KiroCliIntegration.registrar_config["args"] reached the model
verbatim and broke the prompt — every parameterized SpecKit
command under Kiro CLI was unusable.

Replace the placeholder with a prose fallback that instructs
the model to take its argument from the user's next message,
mirroring the convention used by other integrations whose
target CLI lacks native argument injection.

Add two regression tests in TestKiroCliIntegration:
  - test_rendered_prompts_do_not_contain_raw_arguments
  - test_rendered_prompts_contain_kiro_arg_placeholder
and override the inherited test_registrar_config so it does
not require args == "$ARGUMENTS".

Fixes #1926

* test(kiro-cli): tighten args regression guard + document quirk

Address review feedback on PR #2482.

Two changes that bracket the original bug fix from both sides — code AND
documentation:

1. Test layer (Copilot finding at lines 27, 56)

The previous test_registrar_config asserted only that args != "$ARGUMENTS"
and that args is truthy. That would silently pass if a future change
swapped $ARGUMENTS for $INPUT, {{userMessage}}, <args>, or any other
unsubstituted placeholder syntax — defeating the regression guard for
issue #1926.

Replace with a dual-layer guard:

  - test_registrar_config_args_is_exact_prose_fallback pins args to the
    imported _KIRO_ARG_FALLBACK constant. Wording drift now requires a
    deliberate paired commit (production constant + test).

  - test_registrar_config_args_does_not_look_like_a_placeholder_token is
    an independent regression guard built on a 7-pattern regex set
    covering Bash ($X, ${X}, ${X:-default}), Mustache/Handlebars/Jinja
    ({{X}}, {{{X}}}), Liquid/Jinja control ({% %}), Python str.format /
    .NET ({0}, {var}), angle-bracket (<X>), and Windows (%X%). Patterns
    are anchored to the full string so legitimate prose mentioning a
    placeholder ("the {{magic}} of placeholders") is not flagged.

Also fix the line-56 tautology by importing _KIRO_ARG_FALLBACK directly
into test_rendered_prompts_contain_kiro_arg_placeholder, instead of
reading the constant back from registrar_config["args"]. The test now
verifies the FALLBACK STRING reaches the rendered output, independent
of the integration's own config staying correct.

2. Docs layer (mnriem CHANGES_REQUESTED)

The Kiro CLI row in docs/reference/integrations.md only documented its
alias. Update the notes column to lead with the limitation — Kiro CLI
does not substitute $ARGUMENTS in file-based prompts, so Spec Kit ships
a prose fallback at render time — with inline links to upstream Kiro
"Manage prompts" docs and issue #1926. Style follows the Pi row
("limitation first, alias preserved at end").

Refs #1926