feat(FN-4069): add direct merge commit routing to merger

Adds direct merge commit routing to the merger, allowing non-squash merges to bypass the squash-audit path when configured. The feature includes new `mergeCommitStrategy` settings, updated dashboard UI, expanded merger lifecycle tests, and documentation.

Fusion-Task-Id: FN-4069

docs/settings-reference.md

@@ -182,8 +182,27 @@ Defaults from `DEFAULT_PROJECT_SETTINGS`; key scope from `PROJECT_SETTINGS_KEYS`
 | `overlapIgnorePaths` | `string[]` | `[]` | Optional project-relative file or directory paths to exclude from overlap blocking (for example `docs` or `generated/openapi.json`). Entries are trimmed, deduplicated, and must not be absolute or contain `..` traversal. |
 | `autoMerge` | `boolean` | `true` | Auto-finalize tasks from `in-review`. |
 | `mergeStrategy` | `"direct" \| "pull-request"` | `"direct"` | Completion mode (local direct merge vs PR-first). |
+| `directMergeCommitStrategy` | `"auto" \| "always-squash" \| "always-rebase"` | `"auto"` | Direct-merge commit routing mode. `auto` keeps the legacy squash path for branches with zero or one substantive commit, but switches multi-substantive direct merges to a history-preserving rebase-and-merge/cherry-pick path so commit boundaries, subjects, and `Fusion-Task-Id` trailers survive on `main`. `always-squash` forces the legacy squash path; `always-rebase` always preserves per-commit history. Only applies when `mergeStrategy="direct"`. |
 | `mergeConflictStrategy` | `"smart-prefer-main" \| "smart-prefer-branch" \| "ai-only" \| "abort"` | `"smart-prefer-main"` | Controls the merger's conflict-resolution cascade. `smart-prefer-main` fast-forwards local main from `origin` when possible, then tries AI resolution, then auto-resolve heuristics, then a final `-X ours` fallback that prefers main unless the overlap guard below says otherwise. `smart-prefer-branch` uses the same cascade but ends with `-X theirs` so the task branch wins. `ai-only` never silently picks a side, and `abort` stops after the first AI attempt. Legacy `smart` / `prefer-main` values are normalized automatically. |
 | `mergeStrategyOverlapBehavior` | `"flip-to-prefer-branch" \| "warn-only" \| "ignore"` | `"flip-to-prefer-branch"` | Safety control for `mergeConflictStrategy="smart-prefer-main"`. Before the Attempt 3 `-X ours` fallback, Fusion checks whether the task branch and recent `main` history overlap on the same files (30-commit lookback, matching the squash audit heuristics). `flip-to-prefer-branch` makes overlapping files prefer the task branch so hardening is not silently discarded (the FN-3936 class of regression). `warn-only` logs the overlap but keeps the legacy main-wins fallback. `ignore` disables the overlap guard and preserves legacy behavior exactly. |
+
+### Per-task direct-merge override
+
+When a project uses `mergeStrategy: "direct"`, an individual task can override the project-level `directMergeCommitStrategy` by adding this line anywhere in `PROMPT.md`:
+
+```md
+**Direct Merge Commit Strategy:** auto
+```
+
+Accepted values:
+- `auto` — squash if the branch has 0–1 substantive commits; preserve per-commit history if it has 2+
+- `always-squash` — force the legacy squash path for this task
+- `always-rebase` — force the history-preserving path for this task
+
+Override precedence for direct merges is:
+1. Task `PROMPT.md` line `**Direct Merge Commit Strategy:** ...`
+2. Project `directMergeCommitStrategy`
+3. Default `"auto"`
 | `pushAfterMerge` | `boolean` | `false` | Auto-push to remote after successful direct merge. Includes pulling latest and AI conflict resolution. |
 | `pushRemote` | `string` | `"origin"` | Git remote (and optional branch) to push to after merge. |
 | `worktreeInitCommand` | `string` | `undefined` | Shell command run after worktree creation. For pnpm repos, prefer `pnpm install --frozen-lockfile` for deterministic bootstrap. |