feat(python): wire custom builtins through BuiltinRegistry for parity with JS bindings #1724

Context

PR #1721 added BuiltinRegistry to the bashkit core and wired it through the JS bindings: Bash.addBuiltin() / Bash.removeBuiltin() and BashOptions.customBuiltins that survive reset() and don't disturb the VFS.

The Python bindings still use the older build-time-only approach (introduced in #1315). BashOptions.custom_builtins is honored at construction but goes through BashBuilder::builtin(...) — there's no Bash.add_builtin() method, and entries do not flow through the new BuiltinRegistry.

This means Python lacks:

• Post-construction registration / removal of custom builtins
• The "register a builtin after execute() without rebuilding the interpreter" guarantee
• Parity with the JS-side semantics

Breaking changes are acceptable here — Python custom_builtins is a small surface and we'd rather have one consistent design across bindings than carry two.

Scope

• Replace apply_custom_builtins_to_builder (crates/bashkit-python/src/lib.rs:2668) with builder.builtin_registry(registry.clone()) + populate the registry with constructor-time entries
• Add Bash.add_builtin(name, callback) and Bash.remove_builtin(name) methods
• Mirror for BashTool if it exposes a similar API
• Introduce PyCustomBuiltinAdapter (same shape as JsCustomBuiltinAdapter in crates/bashkit-js/src/lib.rs)
• Decide on the sync/async story — Python callbacks are sync by default; the adapter should refuse rather than deadlock when invoked from a sync interpreter path (see related issue for the JS guardrail)
• Update tests in crates/bashkit-python/tests/ to cover post-construction registration, VFS persistence, reset() survival, override precedence
• Update specs/builtins.md Python section if needed
• Update Python-side documentation / examples

References

• PR feat(core,js): host-owned mutable BuiltinRegistry #1721 — JS-side implementation as the reference model
• spec: specs/builtins.md (BuiltinRegistry section)
• core: crates/bashkit/src/builtins/mod.rs (BuiltinRegistry definition)
• JS adapter: crates/bashkit-js/src/lib.rs (JsCustomBuiltinAdapter)
• Original Python custom_builtins PR: feat(python): add custom_builtins to Bash and BashTool #1315