fix(installer): resolve url-source custom modules from custom-modules cache (#2323)

* fix(installer): resolve url-source custom modules from custom-modules cache

resolveInstalledModuleYaml previously only searched ~/.bmad/cache/external-modules/,
so modules installed via --custom-source <git-url> (cached at
~/.bmad/cache/custom-modules/<host>/<owner>/<repo>/) could not be located on
re-install runs. This caused warnings during npx bmad-method install:

  [warn] collectAgentsFromModuleYaml: could not locate module.yaml for '<name>'
  [warn] writeCentralConfig: could not locate module.yaml for '<name>'

Adds a fallback that walks the custom-modules cache via _findCacheRepoRoots
(identifying repo roots by .bmad-source.json or .claude-plugin/, not
marketplace.json, so direct-mode modules are also covered), reuses the same
searchRoot candidate-path logic, and matches by the discovered yaml's code
or name field.

Works without needing _resolutionCache to be populated, which fixes the
re-install scenario where no --custom-source flag is passed.

Closes #2312

* fix(installer): enumerate all module.yamls when walking custom-modules cache

A url-source custom-modules repo can host multiple plugins in discovery
mode (e.g. skills/module-a/module.yaml and skills/module-b/module.yaml).
The previous walk used searchRoot which returned only the first match,
so asking for module-b would surface module-a's yaml, fail the code/name
check, and skip the repo entirely — never inspecting module-b.

Splits the candidate-path traversal into searchRootAll (returns every
module.yaml in priority order) and a thin searchRoot wrapper for the
existing single-module fallbacks. The custom-modules walk now iterates
every yaml per repo and matches each against code or name.