feat(skills): add backport bundled skill

[emal-avala]

Summary

Adds `/backport` — cherry-picks a commit or PR onto one or more release branches. One isolated worktree per branch, per-target verification, conflicts that need judgment stop rather than guess.

Guardrails in the prompt:

• one worktree per target branch — doesn't mutate the current tree
• mechanical conflict resolution only (trivial renames, import reorders); anything requiring judgment stops on that branch and surfaces the conflict
• run the target branch's test + lint gate (fix may depend on newer-branch code)
• push to `backport/-onto-` and open a linked PR
• summary table: `target | backport PR or failure | clean or had-conflicts`
• never force-push, never squash without reviewer sign-off, never mark complete if tests failed

Why

Backports across `release/1.14`, `release/1.15`, `release/1.16` are a common post-fix chore. Without a skill, the agent tends to either try everything in the current working tree (contaminating it) or silently skip failing targets. The `batch` skill covers the parallel-worktree workflow for arbitrary changes; `backport` specializes it for `git cherry-pick` with PR creation baked in.

Complements `/batch` (PR #162) for the cherry-pick-and-open-PR flow specifically.

Test plan

• `cargo fmt --all` clean
• `cargo check` passes
• `cargo test -p agent-code-lib --lib skills` (11 tests pass)
• Manual: `/backport PR 123 to release/1.14 and release/1.15` → two worktrees, two backport branches, two PRs linked to chore: release v0.15.2 #123

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