CLI: add `quickchr cp` for file transfer (upload/download)

[mobileskyfi]

ChrInstance.upload() / download() exist (SCP via resolved creds, src/lib/scp.ts), but there is no CLI command for them. Surfaced by the examples rework (#22):

• examples/file-transfer/ is library-only — its .sh is just a "no CLI" note (and it ships no .ps1 for the same reason).
• examples/version-matrix/'s CLI/Python driver can't upload the sample config, so it compares default exports only (the library .ts does upload via upload()).

A thin command wrapping the existing SCP plumbing would close both gaps and let those examples ship a real CLI/Python transfer path instead of a gap note.

Proposed shape

scp-style, bidirectional:

quickchr cp <local> <name>:<remote>     # upload
quickchr cp <name>:<remote> <local>     # download

…or explicit quickchr push / quickchr pull. Uses the instance's resolved credentials (same as upload()/download()); requires the machine running + scp on the host.

Done when

• cp (or push/pull) lands wrapping src/lib/scp.ts
• examples/file-transfer/file-transfer.sh + .ps1 do a real round-trip
• examples/version-matrix CLI driver can upload config/rb5009-arm64.rsc
• examples/COVERAGE.md "Friction found" notes resolved

Refs: #22

[mobileskyfi]

Centrs-side transfer alignment note:

quickchr cp can stay a quickchr convenience wrapper over the existing SCP/password askpass plumbing. That is separate from centrs transfer --quickchr, which should go through centrs's validated transfer surface and target resolver.

For centrs, REST/native transfer can use the REST/native endpoints and RouterOS password from the #71 descriptor. SFTP transfer is different: centrs uses host sftp with batch mode and key-path/agent-compatible auth, so it needs #71 to expose a verified private-key path or equivalent publickey-capable SSH handoff. Password-only SSH should not be advertised to centrs as usable SFTP support.

No need for quickchr to provision transfer-specific RouterOS state for centrs; this is just descriptor clarity around existing core SSH access.