Bundle installer engine cleanup: tryExec extraction + MEMORY migration (#121, #107)

[virtualian]

Bundles two installer-engine issues on a single branch with one commit each for bisectability.

#121 — Promote tryExec into shared engine/exec.ts

Extract the triplicated safe-subprocess helper into a leaf utility module with zero imports from the installer tree. Replaces three copies:

• actions.ts — delete local tryExec, import from ./exec
• detect.ts — delete local tryExec, import from ./exec, pass explicit 5000ms timeout at every call site to preserve the historical detect-probe bound (tryExec default is 30000ms, matching actions.ts)
• repo-url.ts — rewrite readOriginRemote around tryExec, preserving the .git existsSync short-circuit

Zero behavioral change — purely structural consolidation. Originally flagged by the /simplify review in PR #120 (#115 fix) as pre-existing debt and deferred out of that scope.

Commit: cc0fcc5

#107 — Automate MEMORY migration for v4.0.2 → v4.0.3+ upgraders

Ships engine/memory-migration.ts patterned off command-migration.ts (PR #135). Automates the ~/.claude/MEMORY/ → ~/.pai/MEMORY/ relocation that PR #106 documented but left manual.

Semantics:

• Idempotent via marker file at <dest>/STATE/migration.json. Second run sees the marker and noops regardless of source state.
• Refuses ambiguity: if both source and dest contain meaningful content without a marker, emits a diagnostic listing both paths, file counts, and latest mtime, then stops. Merging is a data-integrity decision outside installer scope.
• Atomic move: renameSync first, cpSync + rmSync fallback on EXDEV. Preserves mode and timestamps. Marker written inside the new location after the move.
• Meaningful-content filter: ignores .DS_Store and dotfile cruft so empty stub directories don't trip ambiguity detection.
• Dry-run mode (options.dryRun=true) reports the intended action without touching the filesystem and without writing telemetry.
• Telemetry logged to <paiDir>/MEMORY/LEARNING/SYSTEM/memory-migration-*.json so the outcome surfaces in the learning digest, not stdout where users miss it.

Wiring: Called from runRepository immediately after migrateUserContext and before migratePerPackSymlinks / migratePerPackCommands. Runs on every install path; the idempotent + noop-on-absent-source semantics make this safe for fresh installs.

Tests (engine/memory-migration.test.ts, 8 cases — all passing):

• fresh install → noop-nothing-to-migrate
• source has only .DS_Store → noop
• upgrade → renames source into dest, writes marker, preserves nested files, method: "rename"
• second run → noop-already-migrated regardless of source state
• ambiguity → refused-ambiguous, neither side mutated, telemetry written
• dry-run would-migrate → filesystem untouched
• dry-run would-refuse-ambiguous → no telemetry written
• post-migration telemetry entry exists in LEARNING/SYSTEM/

Commit: dad8df5

Verification

• engine/exec.ts typechecks via bun build --target=bun
• engine/actions.ts, engine/detect.ts, engine/repo-url.ts typecheck clean after refactor
• engine/memory-migration.ts + engine/memory-migration.test.ts typecheck
• bun test engine/memory-migration.test.ts — 8/8 pass
• bun test engine/skill-migration.test.ts — 17/17 pass (regression check)
• bash -n Releases/v4.0.3+/.claude/install.sh
• bash -n Releases/v4.0.3+/.claude/PAI-Install/install.sh

Test plan

• Maintainer review of Promote tryExec to shared engine/exec.ts — currently triplicated across installer #121 structural change (tryExec extraction)
• Maintainer review of Automated MEMORY migration shim for v4.0.2 → v4.0.3+ upgraders #107 semantics (ambiguity refusal, marker format, telemetry path)
• Manual dry-run on a populated ~/.claude/MEMORY/ if desired: import migrateMemoryDirectory, call with { dryRun: true }, verify no writes

Issues closed

• Closes Promote tryExec to shared engine/exec.ts — currently triplicated across installer #121
• Closes Automated MEMORY migration shim for v4.0.2 → v4.0.3+ upgraders #107

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