feat: skip caching tasks that modify their inputs

[branchseer]

Summary

Tasks like formatters (prettier --write) and auto-fix linters (eslint --fix) read and write the same files. Caching these tasks doesn't work — the stored input hash goes stale after every run, causing the cache to always miss next time.

We now detect when a task reads and writes the same file (via fspy) and skip caching it entirely. The user sees a clear message explaining why.

Also, files already covered by explicit glob inputs (input: ["src/**"]) are no longer fingerprinted again in the post-run check, saving storage and I/O.

Output examples

Single task that modifies its input:

---
vp run: app#format not cached because it modified its input. (Run `vp run --last-details` for full details)

Multiple tasks, one modifies its input:

---
vp run: 1/2 cache hit (50%), 102ms saved. app#format not cached because it modified its input. (Run `vp run --last-details` for full details)

Multiple tasks, several modify their inputs:

---
vp run: 1/3 cache hit (33%), 85ms saved. app#format (and 1 more) not cached because they modified their inputs. (Run `vp run --last-details` for full details)

--verbose / --last-details shows the specific file in the full summary:

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
    Vite+ Task Runner • Execution Summary
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Statistics:   1 tasks • 0 cache hits • 1 cache misses
Performance:  0% cache hit rate

Task Details:
────────────────────────────────────────────────
  [1] app#format: ~/packages/app$ prettier --write src/index.ts ✓
      → Not cached: read and wrote 'packages/app/src/index.ts'
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Test plan

• cargo test -p vite_task — unit tests
• cargo test -p vite_task_bin --test e2e_snapshots — E2E snapshot tests covering:
  • Single read-write task (separate read+write opens)
  • Single O_RDWR open (single syscall)
  • Multi-task with mixed cacheable and non-cacheable tasks
  • Verbose output showing the overlapping path
• cargo clippy -- -D warnings — no new warnings

🤖 Generated with Claude Code

[branchseer]

• feat: skip caching tasks that modify their inputs #248 👈 (View in Graphite)
• main

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