fix(packs): pack registry presets now load at query time

[igor-ctrl]

Summary

Follow-up bug fix surfaced while building the Beautech bootstrap packs.

bcli pack install wrote registries/<profile>.json endpoints as a name→body object, but the runtime EndpointRegistry (src/bcli/registry/_registry.py:52) iterates endpoints as a JSON array. Two consequences:

1. Pack-installed registry presets never resolved at query time (the loader saw an empty/incompatible structure).
2. Installing a pack on top of an existing array-shaped registry — e.g. one produced by bcli registry import — raised InstallError: 'endpoints' must be an object (caught gracefully, no corruption, but presets didn't land).

The runtime loader is the established contract, so the installer now conforms: install and uninstall read/merge/filter endpoints as a list keyed by each entry's entity_set_name.

Changes

• _installer.py install: merge presets into the endpoints array, replacing an entry with the same entity_set_name or appending.
• _installer.py uninstall: filter the array by entity_set_name instead of deleting dict keys.
• New regression test test_installed_preset_is_loadable_by_runtime_registry — installs a preset, then loads it through the real registry loader (EndpointRegistry.load_custom_from_file + .resolve). This is the path that was silently broken; the prior tests only round-tripped through the installer's own (wrong) dict shape.
• Updated the three existing installer-test assertions from dict-shape to array-shape.
• Patch bump 0.6.0 → 0.6.1.

Test plan

• uv run pytest — 947 passed, 5 skipped, 0 failed
• uv run ruff check src/ tests/ — clean
• New round-trip test fails on the old dict-writing code, passes now
• Conflict-detection + ledger paths unchanged (ledger still keys by preset name)