Add amd64 arch mapping, tar pre-check, and gzip SCP fallback

[alokedesai]

Description

Problem: Three related install script issues:

1. amd64 arch rejected (7 errors): Some hosts report amd64 from uname -m instead of x86_64 (FreeBSD-style, some WSL configs). Both the shell script and Rust-side parse_uname_output() reject this as unsupported, even though amd64 IS x86_64.
2. tar not found (6 errors): Minimal Docker images and embedded systems lack tar. The install script downloads the full tarball before discovering tar is missing, wasting bandwidth and time.
3. No fallback for tar-less hosts: When tar is missing, the install fails with no recovery path.

Solution:

1. amd64 mapping: Add "amd64" to the x86_64 match arm in both parse_uname_output() and the install script case statement.
2. tar pre-check: Add command -v tar check in the install script BEFORE downloading. Exits with NO_TAR_EXIT_CODE = 4 so the Rust side can handle it distinctly.
3. gzip SCP fallback: New gzip_scp_install_fallback() function that downloads the tarball locally, extracts the binary locally with tar, gzips just the binary, SCPs the .gz to the remote, and runs gzip -d && chmod +x via SSH. This only requires gzip on the remote (present on virtually all Linux systems including minimal Docker images).

Linked Issue

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below.

Testing

Unit tests (all passing):

• cargo test -p remote_server -- setup::tests — 26 passed, 0 failed
• cargo fmt / cargo clippy — clean

New tests:

• parse_uname_linux_amd64 — verifies "Linux amd64" maps to RemotePlatform { os: Linux, arch: X86_64 }
• parse_uname_unsupported_armv7l — verifies "Linux armv7l" returns UnsupportedArch
• install_script_contains_no_tar_exit_code — verifies {no_tar_exit_code} placeholder is substituted
• install_script_contains_tar_pre_check — verifies command -v tar is present in script

Integration tests (shell-based, 7/7 pass):

• OLD script rejects amd64 with "unsupported arch: amd64" -> NEW script maps it to x86_64
• OLD script fails late without tar ("tar: command not found") -> NEW script exits early with code 4
• gzip decompression + chmod + execution verified on simulated tar-less environment

Impact: Addresses unsupported arch amd64 (0.7%), tar not found (0.6%)

• I have manually tested my changes locally with ./script/run

Agent Mode

• Warp Agent Mode - This PR was created via Warp AI Agent Mode

Co-Authored-By: Oz oz-agent@warp.dev

CHANGELOG-IMPROVEMENT: SSH extension now supports amd64 architecture and can install on hosts without tar via gzip fallback