Hook insertion in add_repo() used hand-rolled precedent/successor logic against a static _HOOK_ORDER list. This replaces it with pipeweld's Adder, which models the same ordering constraints as a series-parallel graph — laying groundwork for repo-aware insertion and prek priority support.

Changes

• func.py — Adder: Added force_linear setting that post-processes the solution to flatten any Parallel nodes into a deterministic linear Series. Added _extract_ordered_steps() and _linearize_component() helpers internal to the pipeweld layer.
• func.py — get_predecessor(): Added a public function that recursively walks the full component tree (str, Series, Parallel, DepGroup, nested structures) to find the step that immediately precedes a given step. Returns None if the step is first, raises ValueError if not found.
• hooks.py — add_repo(): Build a Series pipeline from existing hook IDs, derive prerequisites/postrequisites sets from _HOOK_ORDER, delegate insertion positioning to Adder.add() with force_linear=True, and use get_predecessor() to extract the predecessor from the solution.
• test_func.py: Unit tests for _extract_ordered_steps, _linearize_component, Adder with force_linear=True, and get_predecessor() with comprehensive coverage across all component types and nesting depths (32 new tests)
• test_hooks.py: Integration tests exercising pipeweld-based insertion (prerequisite present, postrequisite only, mixed non-dependent/postrequisite)

Before/After

# Before: manual list slicing
precedents = _HOOK_ORDER[:hook_idx]
successors = _HOOK_ORDER[hook_idx + 1:]
existing_precedents = [h for h in existing_hooks if h in precedents]
# ... 15 lines of if/elif/else to find last_precedent

# After: pipeweld solver
adder = Adder(
    pipeline=series(*existing_hooks),
    step=hook_config.id,
    prerequisites=set(_HOOK_ORDER[:hook_idx]),
    postrequisites=set(_HOOK_ORDER[hook_idx + 1:]),
    force_linear=True,
)
result = adder.add()
predecessor = get_predecessor(result.solution, hook_config.id)

All existing tests pass unchanged — behavioral compatibility is preserved for current hook orderings.

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