fix: replace broad except Exception with specific exception types

[raiderrobert]

Addresses #46.

See #46 for context.

What changed

• src/napoln/core/resolver.py: Replaced except Exception with (OSError, yaml.YAMLError, UnicodeDecodeError) in _extract_version and _extract_description.
• src/napoln/core/merger.py: Replaced except Exception with (OSError, UnicodeDecodeError) in has_conflict_markers.
• src/napoln/core/manifest.py: Replaced except Exception with (OSError, tomllib.TOMLDecodeError) in read_manifest; replaced except Exception with except OSError in write_manifest temp cleanup.
• src/napoln/core/store.py: Replaced except Exception with except OSError in store_skill temp cleanup.
• src/napoln/commands/add.py: Replaced except Exception with (StoreError, OSError) for store, (PlacementError, OSError) for placement, and (ManifestError, OSError) for manifest reads in --skill picker.
• src/napoln/commands/install.py: Replaced except Exception with (PlacementError, OSError) in _sync_manifest.
• src/napoln/commands/enable.py: Replaced except Exception with (StoreError, OSError) and (PlacementError, OSError) in _place_skill_for_agent.
• Added regression tests for all changes verifying expected failures are caught and programming bugs (e.g. TypeError) propagate uncaught.

How to verify

uv run pytest
uv run ruff check src/ tests/