Keep upload file names and readable bytes for file inputs

[wolfiesch]

Problem

upload_file failed on sites that check uploads, such as GitHub's avatar upload:

1. The host staged each upload as <uuid>.upload, so the page saw a file with no extension or MIME type and rejected it ("Only images, please").
2. The host deleted the staged copy as soon as the action returned. Chrome hands the page a path-backed File and reads the bytes only when the page uses it, so the upload request then failed.

Change

• Stage each upload as upload-staging/<uuid>/<source file name> (directory 0700, file 0600). Cleanup removes the file and its per-upload directory.
• Uploads that reached the page (direct act or committed) are removed 10 minutes later. Uploads that never reach the page are still removed immediately.
• On startup, the host removes per-upload directories older than 15 minutes that an exited host left behind. That age is longer than both the retention and the 5-minute staged-commit lifetime.
• Updated docs/security.md and the changelog.

Verification

• cargo test -p agenttab-host: all pass. New tests cover the kept file name, cleanup of the per-upload directory, and the stale sweep. The commit test now asserts the staged bytes stay readable.
• Live check: GitHub App logo upload through the installed host. GitHub accepted the PNG and opened its crop dialog. Before this change, the same upload failed with is-bad-file (from the name fix) and then is-failed (from early deletion).
• cargo clippy -D warnings reports only the two too_many_arguments errors already on main (journal.rs, runtime.rs).

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