fix: resolve command references per integration type (dot vs hyphen) (#2354)

* fix: resolve command references per integration type (dot vs hyphen)

Replace hardcoded /speckit.<cmd> references in templates with
__SPECKIT_COMMAND_<NAME>__ placeholders that are resolved at
setup time based on the integration type:

- Markdown/TOML/YAML agents: separator='.' → /speckit.plan
- Skills agents: separator='-' → /speckit-plan

Changes:
- Add resolve_command_refs() static method to IntegrationBase
- Add invoke_separator class attribute (.  for base, - for skills)
- Wire into process_template() as step 8
- Update _install_shared_infra() to process page templates
- Replace /speckit.* in 5 command templates and 3 page templates
- Add unit tests for resolve_command_refs (positive + negative)
- Add integration tests verifying on-disk content for all agents
- Add end-to-end CLI tests for Claude (skills) and Copilot (markdown)

Fixes #2347

* review: use effective_invoke_separator() for Copilot skills mode

Address PR review feedback: instead of bleeding _skills_mode
knowledge into the CLI layer, add effective_invoke_separator()
method to IntegrationBase that accepts parsed_options.

CopilotIntegration overrides it to return "-" when skills
mode is requested. The CLI layer simply asks the integration
for its separator — no hasattr or _skills_mode coupling.

Also adds tests for the new method on both base and Copilot,
plus an end-to-end test for 'specify init --integration copilot
--integration-options --skills' verifying page templates get
hyphen refs.

* fix: build_command_invocation preserves full suffix for extension commands

Previously rsplit('.', 1)[-1] on 'speckit.git.commit' yielded
just 'commit', producing /speckit.commit instead of
/speckit.git.commit (or /speckit-git-commit for skills).

Fix: strip only the 'speckit.' prefix when present, then join
remaining segments with the appropriate separator.

Updated in IntegrationBase, SkillsIntegration, and
CopilotIntegration. Added tests for extension commands in
build_command_invocation across all three.

* fix: Copilot dispatch_command() preserves full extension command suffix

dispatch_command() had the same rsplit('.', 1)[-1] bug as
build_command_invocation() — speckit.git.commit would dispatch
as /speckit-commit instead of /speckit-git-commit in skills
mode, or --agent speckit.commit instead of speckit.git.commit
in default mode.