add interactive microservice-selection checklist to install wizard

[HumanBean17]

What

Implements the interactive microservice-selection checklist from propose/completed/CLI-INSTALL-PROPOSE.md Stage 1 (Case B) — a step that was specified in the proposal but never built. Previously, java-codebase-rag install auto-selected every detected microservice with no user interaction, so a user installing into a microservice monorepo had no way to exclude services.

User-visible changes

• When install detects 2 or more microservices (a monorepo where cwd has no root build file but multiple subdirectories contain pom.xml / build.gradle*), it now shows an interactive checklist (questionary.checkbox) with all services pre-checked. Toggle off the ones to exclude (Space), confirm with Enter:
  • Select all → microservice_roots is omitted from .java-codebase-rag.yml (the indexer walks the entire tree — equivalent to accepting the default).
  • Select a subset → only those roots are written to microservice_roots.
  • Deselect everything → "At least one module is required. Re-select?" (Yes retries, No aborts with exit code 2).
• Single-module projects (root has its own build file) and single-service detection skip the checklist — there's nothing to choose.
• On re-run (install into an existing config, choosing Update), the previously-selected microservice_roots subset is pre-checked, so the user can adjust rather than start over.

Behavior change to note

In non-interactive mode (--non-interactive or piped stdin), a multi-service install previously wrote an explicit full microservice_roots list to the YAML; it now omits the key entirely. Both index everything — the new behavior just matches the proposal's "accept default → omitted" rule and produces cleaner YAML. No test asserted the old shape.

Scope / impact

• CLI wizard only. No --microservice flag added — non-interactive keeps the "index all" default per the proposal's non-interactive section.
• No re-index required. No env-var, ontology, or schema changes.

Tests

• New TestSelectMicroservices (7 tests): non-interactive → None; non-TTY → None (default=all); subset selection; all-selected → None; empty + decline → exit 2; re-run preselected marks the right checked choices; single dir → None.
• tests/test_installer.py: 73 passed. Full suite: 760 passed, 11 skipped. ruff check clean.

Manual verification (interactive TTY — can't run under CI)

In a real terminal from a multi-service fixture dir (service-a/pom.xml, service-b/pom.xml, service-c/pom.xml, no root build file):

java-codebase-rag install

• Confirm source root (Enter) → microservice checklist appears, all checked.
• Toggle one off (Space), Enter → .java-codebase-rag.yml lists only the selected subset under microservice_roots.
• Re-run install, choose Update → the previously-unchecked service starts unchecked.
• Re-run, uncheck all, answer No to re-select → exit code 2.

Non-interactive end-to-end (proves the wiring): java-codebase-rag install --non-interactive --agent claude-code → .java-codebase-rag.yml has no microservice_roots key.

Implements propose/completed/CLI-INSTALL-PROPOSE.md Stage 1 (Case B).

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