fix(snapshot): store function source alongside ast

[chaliy]

What

• store function source alongside serialized AST in shell snapshots
• restore functions from source when AST is missing
• add exclude_functions snapshot option so prompt-oriented snapshots can skip cloning function state
• expose the new option in Python and JS bindings and document it
• update the local ship skill to create a branch when shipping from detached HEAD

Why

• AST-only function snapshots are brittle across parser/schema evolution
• prompt/state inspection flows should avoid cloning AST-heavy function maps when they do not need function restore

How

• capture original function source during parse
• serialize snapshots as source plus AST, with source fallback on deserialize
• thread exclude_functions through Rust, Python, and JS snapshot APIs
• add regression tests for source-only restore and function-free snapshots

Validation

• cargo test -p bashkit --test snapshot_tests
• cargo test -p bashkit-python --no-run
• cargo test -p bashkit-js --no-run
• just test
• just pre-pr

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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