integration: Add AI-driven conflict resolution

[ed-irl]

integration: Add AI-driven conflict resolution

Adds an opt-in fallback that invokes a user-configured shell script
when an integration-rebuild merge conflicts. Rerere and the regenerate
merge driver already handle the mechanical conflicts; this is for
the residue.

The resolver follows a strict JSON protocol on stdout:
{"assumptions": [...], "questions": [...], "unresolved_files": [...]}
Empty (or assumptions-only) means "all resolved." Questions trigger
an interactive Q&A loop whose pairs persist to .integration_resolution.json
at the repo root. unresolved_files with no questions surfaces as an
error for manual investigation. Non-zero exit or invalid JSON falls
through to the existing conflict-surfacing path.

The resolution file accumulates Q&A across rebuilds so the user
doesn't re-answer for already-resolved (ours, theirs) pairs. Entries
are pruned when branches are untracked, deleted, or removed by sync.

Scoped exclusively to gs integration rebuild and the MaybeRebuild
hook on restack — no impact on branch_merge, restack, or any
user-visible merge. Blast radius: one throwaway branch.

Configured via spice.integration.resolver (script) and
spice.integration.autoResolve (default behavior); --auto-resolve /
--no-auto-resolve override per invocation. Iteration cap is 10 to
prevent runaway loops.

integration: AI-driven auto-resolve for merge conflicts

Adds an auto-resolve path for integration-rebuild merge conflicts.
When configured and enabled, git-spice invokes a user-supplied
script on every merge conflict and, if the script reports the
conflict resolved, stages the files and commits the merge without
intervention.

Shared with the rest of git-spice's script-driven features via the
protocol surface on ed-irl/scriptrun:

• Uses scriptrun.ResolveResponse for the JSON output schema and
  scriptrun.QAPair for resolution-file entries -- no duplicated
  types.
• Passes the shared GS_OPERATION (always "integration-rebuild" for
  this feature) / GS_BRANCH (the tip being merged) / GS_BASE (the
  integration branch) env via scriptrun.EnvFor.
• Iteration cap reads spice.scriptResolve.maxIterations
  (default 10) via Handler.MaxResolveIterations, replacing the
  previously private constant. main.go wires it through.
• Resolution file lives at .spice/resolutions/integration.json via
  spicedir; the old .integration_resolution.json root-level path
  is gone. Save() auto-creates the parent directory.
• Resolution-file unknown-field rejection is dropped: per the
  shared protocol, extra fields are ignored.
• Per-protocol details (env contract, JSON schema, persistence,
  iteration cap, fall-through) are documented once in
  doc/src/guide/scripts.md; doc/src/guide/integration.md is
  trimmed to defer there and only documents integration-specific
  UX (Claude Code recipe, the GS_OPERATION value, resolution-file
  key layout).

CLI:

• gs integration rebuild accepts --auto-resolve /
  --no-auto-resolve, with the default coming from
  spice.integration.autoResolve.
• The resolver is configured via spice.integration.resolver
  (script body).

Tests:

• Existing success / disabled / assumptions / invalid_output /
  unresolved_no_questions scripts updated to use the new
  .spice/resolutions/integration.json path.
• persists_across_rebuilds, prune_on_delete, prune_on_untrack
  learned to mkdir the resolution dir before seeding.
• Two new scripts (env, iteration_cap) lock the shared env
  contract and the configurable iteration cap (via
  spice.scriptResolve.maxIterations).

[ed-irl]

This change is part of the following stack:

• restack: Add Options parameter to high-level restack entry points #1307
  • feat: Add worktree commands with per-worktree trunks #1249
    • feat(worktree): Add worktree commands, per-worktree trunks, and worktree-aware filtering #1188
      • worktree: Add worktree commands, per-worktree trunks, and worktree-aware filtering #1165
        • feat: Add worktree commands, per-worktree trunks, and worktree filters #1172
          • worktree: Add worktree management commands, per-worktree trunks, and worktree-aware filtering #1173
            • integration: Wire auto-rebuild hook on restack commands #1174
              • integration: Auto-rebuild integration branch after restack #1175
                • integration: Auto-rebuild integration branch on restack #1176
                  • integration: Add AI-driven conflict resolution #1251 ◀
                  • integration: Move regeneration out of merge driver #1252
                  • integration: accept-incoming fallback after auto-resolve #1253
                  • integration: submit tip downstacks alongside the integration branch #1254
                  • integration: surface integration branch and tips in gs ll #1255
                  • integration: short-circuit gs branch checkout for the integration #1256
                  • integration: gs ss from integration submits all tip stacks #1257
                  • integration: tip clean prunes subsumed tips #1258
                  • integration: tip advance moves tips to upstack leaves #1259
                  • integration: rebuild fixes and reliability improvements #1260

_{Change managed by git-spice.}