Assert every command placeholder names a command that exists

[Hermann-web]

Problem

IntegrationBase.resolve_command_refs rewrites __SPECKIT_COMMAND_<NAME>__
with a bare regex and never consults a list of commands:

return re.sub(
    r"__SPECKIT_COMMAND_([A-Z][A-Z0-9_]*)__",
    lambda m: prefix + "speckit" + separator
    + m.group(1).lower().replace("_", separator),
    content,
)

Any name that matches the pattern is rewritten. A typo such as
__SPECKIT_COMMAND_ASSES_SHAPE__ becomes /speckit.asses.shape, which is
written into a user's agent instructions as a plausible-looking command that no
agent can run. There is no error, and the failure surfaces only when someone
tries the command.

Existing coverage checks the mechanism, not the target:

• tests/integrations/test_base.py asserts the separator variants are produced;
• tests/test_agent_config_consistency.py asserts extension tokens resolve with
  the correct separator.

Neither asserts that the resolved command exists. This matters more now that
extensions/catalog.community.json carries 157 community extensions whose
authors are told to use these placeholders.

Change

One new test module, tests/test_command_placeholder_targets.py. No production
code changes.

The command set is derived rather than restated:

• speckit.<stem> for every templates/commands/*.md;
• every provides.commands[].name and aliases from each
  extensions/*/extension.yml and presets/*/preset.yml.

A command added or renamed is therefore covered without editing the test.

It asserts:

1. Every placeholder in shipped markdown (templates/, extensions/,
   presets/) resolves to a command that exists.
2. Core templates do not reference a command only an extension provides, since a
   core template has to work when that extension is not installed.
3. Placeholders were actually found, so a change to the token spelling or to the
   shipped directories fails loudly instead of scanning nothing and passing.
4. The mapping used by the test is the one resolve_command_refs implements,
   checked against the real function rather than reimplemented on faith.

Verification

Passes on current main (95efce4): the repository is clean today, with all 17
placeholders resolving and no core template leaking an extension reference.

Both assertions were confirmed to fail before committing, by appending
__SPECKIT_COMMAND_ASSES_SHAPE__ and __SPECKIT_COMMAND_BUG_FIX__ to
templates/commands/plan.md:

AssertionError: placeholders resolve to commands that do not exist:
  /speckit.asses.shape referenced by templates/commands/plan.md
AssertionError: core templates reference commands only an extension provides,
  so the instruction is dead unless that extension is installed:
  /speckit.bug.fix in templates/commands/plan.md

The injection was reverted; the diff is the new file only.

pytest tests/test_command_placeholder_targets.py is 6 passed.
pytest tests/test_agent_config_consistency.py tests/integrations/test_base.py tests/test_commands_package.py tests/test_command_placeholder_targets.py is
125 passed.

The full suite was also run: 6958 passed, 155 skipped, and 26 failures all in
test_ps_* PowerShell parity tests because pwsh is not installed on the
machine used. No non-PowerShell failures.

Scope

Deliberately small and additive, per CONTRIBUTING's note about large unagreed
changes. It adds coverage for an existing behaviour and changes nothing that
ships to users.