[ITG] Implement ITG cache to store reverse dependencies of targets

[yushan8]

Why?

Implement ITG cache to store reverse dependencies of targets. Storing the ITG graph allows us to search for the commit closest to the base commit and incrementally recompute the target graph of the commit if it's ITG applicable.
The cache is stored in a separate folder by date so when we load the list of cached ITG graphs, it will only load all the entries for the day.

What?

core/storage — Added List(ctx, dir) to the Storage interface

• storage.go — Added List(ctx context.Context, dir string) ([]string, error) to the Storage interface

• memstorage.go — Filters keys by strings.HasPrefix(key, dir)

• disk/disk.go — Walks rootDir/dir, returns paths relative to rootDir; non-existent directory returns empty without error

• storagemock/storagemock.go — Added List mock method and recorder

• changedtargetsandedgesreader_test.go — Updated errStorage and nilResponseStorage stubs to satisfy the new interface
  Storage-backed cache for OptimizedGraph entries, keyed by (remote, commit_timestamp, sha) and stored under
  itg/{remote}/{date}/{timestamp}_{sha}.

• Put — gob-encodes the graph and writes to storage; skips if the key already exists

• Get — downloads and gob-decodes the graph

• FloorKey — lists all keys under the remote's prefix, parses timestamps, and binary-searches for the latest entry with commit_timestamp ≤ targetTimeSecond

Test Plan

unit tests

Issue

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