feat: add P1 multi-base delivery planning

[Wibias]

Summary

• add head-bound multi-base delivery planning for backports/ports
• create one independent parallel-port identity per target base rather than treating ports as a stacked PR chain
• bind each port to repository, source PR, exact source head SHA, and target base through deterministic provenance markers
• keep all requested targets required by default, with explicit optional-target support
• fail closed on invalid target refs, wrong-base provenance, and duplicate port PRs for one target
• report multi-base delivery complete only when every required port is verified merged
• route backport requests through a dedicated maintainer workflow while preserving separate explicit merge authority
• expose backport/port delivery in SKILL.md discovery metadata

Hardening found during self-review

• a PR carrying the correct provenance marker on the wrong base is surfaced as invalid evidence instead of disappearing as merely missing
• one PR carrying multiple port provenance markers is explicitly invalid/ambiguous evidence
• generated port branches include a short hash of the exact target base, so different refs with the same readable slug cannot collide
• target refs reject leading - in addition to other invalid git-ref forms
• child routing and helpers preserve feat: add P1 work-item delivery orchestration #284/feat: add P1 PR consolidation analysis #285/feat: add P1 conditional visual review evidence #286 hardening during restacks

Stack

• depends on feat: add P1 conditional visual review evidence #286
• feat: add P1 conditional visual review evidence #286 depends on feat: add P1 PR consolidation analysis #285 -> feat: add P1 work-item delivery orchestration #284 -> feat: add P0 open-work and publication safety #283

Validation

• focused contracts cover port planning, git-style ref validation, collision-safe branch identities, provenance binding, wrong-base rejection, multi-marker ambiguity, duplicate ports, required/optional completion, routing, merge-authority separation, mutation modes, and controller phases
• repository-wide local suite is intentionally left to the user's local run; GitHub Actions is not used as the current gate

Provenance

The multi-target delivery direction was informed by OutThisLife/brooklyn-skills delivery patterns and redesigned around GitHub Delivery's current-head, publication, merge-authority, and stacked-PR boundaries.

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