Fix: Narrow INSTALL_ERROR_TYPES catch scope

[kasperjunge]

INSTALL_ERROR_TYPES in exceptions.py catches broadly:

INSTALL_ERROR_TYPES = (FileExistsError, AgrError, OSError, ValueError)

Catching OSError and ValueError broadly risks swallowing unexpected errors and weakens the value of the exception hierarchy.

Proposal

1. Replace bare ValueError raises within install/sync paths with specific AgrError subclasses (e.g., InvalidHandleError, ConfigError)
2. Narrow OSError to specific subclasses (PermissionError, FileNotFoundError) where the failure mode is known
3. Audit all raise sites that currently raise ValueError within the install/sync code paths and convert to the appropriate AgrError subclass

Goal state

INSTALL_ERROR_TYPES = (FileExistsError, AgrError, PermissionError, FileNotFoundError)

Or ideally just (AgrError, FileExistsError) if all ValueError and OSError sites are converted.

Affected files

• agr/exceptions.py — INSTALL_ERROR_TYPES definition
• agr/fetcher.py — primary consumer of the catch tuple
• agr/commands/sync.py — uses INSTALL_ERROR_TYPES in except clauses
• agr/commands/add.py — uses INSTALL_ERROR_TYPES in except clauses
• Various modules that raise ValueError in install paths

Context

Identified in architecture audit: docs/contributing/architecture-audit.md