Fix initial watch sync for existing files - #14031

What I did

Fixes #13725.

develop.watch initial sync filtered host files by the service image's creation time. Existing files older than the image were therefore omitted at startup, even though later filesystem events synced them normally.

Initial sync now copies every eligible watched file, while preserving ignore and bind-mount filtering. The obsolete image-time lookup is removed.

Test verification

RED on upstream main (832673c8e445b5bf0bdb4963c7d7b3f81aa46da0):

TestInitialSyncFilesIncludesFilesOlderThanImage
paths: nil
expected: /app/src/test.txt
--- FAIL

GREEN with this change:

--- PASS: TestInitialSyncFilesIncludesFilesOlderThanImage
--- PASS: TestInitialSyncIncludesSingleFileOlderThanImage

The live Engine reproduction also changed from No such file or directory to successfully reading hello from /app/src/test.txt at watch startup.

Validation

Full local validation suite replayed on upstream and this branch:

• golangci-lint run --build-tags e2e ./... — 0 issues
• pkg/compose and all baseline-green unit packages pass
• changed-line coverage gate passes
• the same two container/inotify-dependent pkg/watch tests fail on both upstream and this branch; no new failures