local init --json: report all created paths

[sdairs]

Summary

• local init --json previously reported only {"path": ".clickhouse/"}, omitting the clickhouse/ and postgres/ scaffold subtrees that the human-readable output already reports (e.g. Created project scaffold in clickhouse/ (tables, materialized_views, queries, seed)).
• init() now returns an InitResult indicating which of .clickhouse/, clickhouse/, and postgres/ were actually created during the invocation.
• InitOutput now exposes paths: Vec<String> (replacing the single path: String field) listing .clickhouse/ always, plus clickhouse/ and/or postgres/ only when their scaffold was newly created this run — matching the conditional human-readable messages exactly.
• Human Display output for InitOutput also grew a line per newly-created scaffold, so local init (non-JSON) now reports scaffold creation via the same output type, not just via eprintln! inside init().

This PR is part of a stacked chain based on fix/607-prometheus-json-help (not main).

Test plan

• cargo fmt --all --check
• cargo clippy -p clickhousectl --all-targets -- -D warnings
• cargo test -p clickhousectl (all passing, including new tests/local_init_json_test.rs subprocess coverage for first-run vs. idempotent-run JSON payloads, and updated inline unit tests in src/init.rs / src/local/output.rs)

Fixes #609

🤖 Generated with Claude Code

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