feat: record stacked base locally so gw status suggests -B

[zawakin]

What

Phase 2 of stacked-PR support. gw new --stack now records the parent in branch.<child>.gwBase (local git config). Before a PR exists, gw status reads it and turns the create-PR next-action into gh pr create -B <parent>, eliminating the one manual step that survived #43's structural redesign — forgetting -B.

Once a PR exists, GitHub's base_branch stays authoritative (unchanged); gwBase only fills the pre-PR gap.

Changes

• git: branch_base / set_branch_base / unset_branch_base helpers (local config, no network).
• new: record the base after a successful --stack.
• next_action: CreatePr { base: Option<String> } — emits -B <parent> when stacked.
• status: read + filter the recorded base (real parent, not the default branch / self), show Base: <parent> (stacked, PR not created yet) pre-PR, pass it to detect.
• sync: unset_branch_base after restacking onto the default branch (no longer stacked). Branch deletion drops the whole [branch] section, so cleanup needs no change (verified).

Rationale for the stale-base decision

A gwBase can go stale only in a narrow window (parent merged + branch deleted between --stack and the child's PR). Phase 2 deliberately does not verify the parent's remote existence:

• gh pr create -B X fails loudly on a missing base (gw never runs it itself — it's a suggested command).
• Verification would add an ls-remote round-trip to every gw status on a stacked branch.
• The correct response to "parent merged" is sync/rebase, which the parent/child guard work (a later phase) will own holistically — dropping -B here would be half-right and mask it.

Test

• next_action: unit test for CreatePr { base: Some(..) }.
• tests/new_test.rs: --stack records gwBase; plain new records none; gw status surfaces the stacked base and the -B feature/parent hint end-to-end (no manual checking needed).
• mise run verify passes.

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