feat(axi): add per-run PR base branch via --base-branch

[lakshyads05]

# Summary
- Add `no-mistakes axi run --base-branch <branch>` to open a PR against a named integration branch for one run only
- Persist the choice on the run record so rebase, PR creation, and CI honor it consistently across resume and rerun
- Validate branch names and remote existence before starting a run

# Changes

## CLI and daemon
- New `--base-branch` flag on `axi run` (and `rerun` for explicit override); value travels through git push options and IPC like intent/skip
- `runs.pr_base_branch` column stores the per-run override for the whole pipeline lifetime

## Pipeline
- `effectivePRBaseBranch` precedence: per-run flag > `pr.base_branch` > repository default
- PR step uses the shared resolver (including skip-when-branch-equals-base)
- Existing PR forge base still wins for CI/ci_fix repair paths (unchanged)

# Why
Teams working several tasks in one repository often need each validation run to target a different epic or feature branch. Repo-wide `pr.base_branch` cannot express that without committing config per task, and retargeting after the fact wastes CI and freezes the wrong PR body. A per-run operator flag extends the existing mechanism without changing default behavior when omitted.

## Precedence
1. Per-run `--base-branch` (operator CLI / push option)
2. Repo `pr.base_branch` (trusted default-branch config)
3. Repository forge default branch
4. For CI merge-conflict repair on an **existing** PR: live forge base from the open PR still wins (unchanged)

## Trust boundary
`pr.base_branch` stays trusted default-branch-only gate control. The new flag is operator-supplied at run start—the same trust path as `--intent` and `--skip`—and does not let pushed branch content choose the integration target.

# Test plan
- [x] `go test -race` on touched packages (`internal/cli`, `internal/daemon`, `internal/db`, `internal/pipeline/steps`) passes
- [x] `make lint` passes
- [ ] Full `go test -race ./...` on maintainer CI (local macOS run hit unrelated `internal/branchsync` git segfault flakes)
- [ ] `axi run --base-branch epic/foo` opens PR against `epic/foo` on a repo with concurrent epic branches

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (3): Last reviewed commit: "fix(pr): retarget Gitea PRs and fail clo..." | Re-trigger Greptile}

[lakshyads05]

Addressed both review notes in 2619601:

1. Existing PR base. A per-run --base-branch that disagrees with an already-open PR now retargets that PR (gh pr edit --base / glab mr update --target-branch) before updating title and body. Repo-config pr.base_branch changes still do not retarget, so an existing PR is updated in place rather than duplicated.

2. Reattach. axi run --base-branch against an in-flight run now refuses when the flag would be discarded. Matching (or omitted) --base-branch still reattaches.

[kunchenguid]

Speaking as Kun's firstmate:

First look at HEAD 26196012 (--base-branch CLI/IPC/DB, effectivePRBaseBranch, existing-PR retarget, reattach refuse-on-mismatch). First-time fork CI approved (CI / docs / Guard).

Diff review: opt-in operator flag on the same trust path as --intent/--skip; omitted = prior behavior. Pushed branch content cannot choose the base. Greptile's earlier notes on retarget + silent discard look addressed in this head. Not a default gate-pass change. Waiting on hosted CI + green Greptile on this SHA before merge.

VISION.md:

• R1: aligns (opt-in per-run targeting; core pipeline order unchanged).
• R2: aligns.
• R3: aligns (operator judgment for integration target).
• R4: aligns (trust boundary stated; forge base for existing-PR CI repair unchanged when no override).
• R5: aligns.
• R6: aligns.
• R7: aligns.

[lakshyads05]

Addressed the Greptile P1 in 9d7ae7c:

• Silent skip. A per-run --base-branch on a provider that cannot retarget now fails closed, including when the live forge base is unread. Title/body are no longer rewritten against a base the PR did not move to.
• Gitea. FindPR copies the listed base, and SetPRBaseBranch PATCHes via tea api (tea pulls edit has no --base). GitHub and GitLab retarget paths are unchanged.

[kunchenguid]

Speaking as Kun's firstmate:

Re-triaged HEAD 9d7ae7c7 after your Gitea/fail-closed retarget follow-up. Greptile 5/5. Fork CI re-approved on this SHA (was action_required again). Opt-in --base-branch; providers that cannot retarget fail closed. Waiting on hosted CI before merge.

VISION.md:

• R1–R7: aligns (unchanged vs prior; fail-closed retarget strengthens R5).