feat: add pixi publish command for building and uploading packages

[wolfv]

Description

This PR introduces a new pixi publish command that combines building and uploading conda packages in a single step. It's a convenience wrapper around pixi build and pixi upload functionality. This is ported from rattler-build.

Key Features:

• Builds conda packages from a workspace manifest
• Automatically uploads to multiple backend types:
  • prefix.dev (with optional sigstore attestation support)
  • Anaconda.org
  • S3-compatible storage (with automatic channel initialization and indexing)
  • Local filesystem (with automatic channel initialization and indexing)
  • Quetz servers
  • JFrog Artifactory
• Supports all standard build options: --target-platform, --build-platform, --build-dir, --clean, --path
• Supports upload options: --force (overwrite existing packages), --generate-attestation (prefix.dev only)
• Reuses existing authentication infrastructure via pixi auth and environment variables
• Automatically detects upload backend from URL scheme or host

Implementation Details:

• Phase 1: Reuses the build logic from pixi build command to compile packages
• Phase 2: Routes to appropriate upload backend based on URL parsing
• Handles platform-specific subdirectories for S3 and local filesystem uploads
• Automatically initializes and indexes channels for S3 and filesystem backends

Documentation:

• Added comprehensive CLI reference documentation with examples for each backend
• Updated prefix.dev deployment guide with pixi publish examples and trusted publishing information
• Included authentication guidance and comparison with separate pixi build/pixi upload workflow

How Has This Been Tested?

The implementation leverages existing, well-tested components:

• Build logic is identical to pixi build command
• Upload backends use rattler_upload and rattler_index libraries (existing dependencies)
• URL parsing and backend detection are straightforward and covered by the routing logic
• Authentication uses the existing pixi_auth infrastructure

Manual testing would involve:

1. Building and publishing to prefix.dev with API key authentication
2. Publishing to local filesystem and verifying channel indexing
3. Testing URL parsing for various backend formats
4. Verifying --force flag behavior with existing packages

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• Changes follow the existing code patterns in pixi_cli (similar to build.rs)

https://claude.ai/code/session_016n3z1PSG2UYV622ZDxwXD6

[wolfv]

Test protocol:

• pixi auth login prefix.dev --token $PREFIX_API_KEY
• pixi publish --to https://prefix.dev/wolf-private-test succesfully published sdl_example to the channel (for osx-arm64).

Local file channel works and creates local index:

╭─wolfv@pixi ~/Programs/pixi/examples/pixi-build/cpp-sdl ‹claude/add-pixi-publish-3FWWs●›
╰─$ cargo r -- publish --to file:///tmp/channel                                                                                                                                                                                                            
✔ Successfully built 'sdl_example-0.1.0-h60d57d3_0.conda'
📦 Publishing 1 package(s) to file:///tmp/channel                                                                                                                                                                                                               
✔ Successfully published 1 package(s) to file:///tmp/channel
╭─wolfv@pixi ~/Programs/pixi/examples/pixi-build/cpp-sdl ‹claude/add-pixi-publish-3FWWs●›
╰─$ ll /tmp/channel
total 0
drwxr-xr-x 3 wolfv  96 Mar 16 15:39 noarch
drwxr-xr-x 7 wolfv 224 Mar 16 15:39 osx-arm64
╭─wolfv@pixi ~/Programs/pixi/examples/pixi-build/cpp-sdl ‹claude/add-pixi-publish-3FWWs●›
╰─$ ll /tmp/channel/osx-arm64
total 20K
-rw-r--r-- 1 wolfv  155 Mar 16 15:39 repodata_shards.msgpack.zst
-rw-r--r-- 1 wolfv  459 Mar 16 15:39 repodata.json
-rw-r--r-- 1 wolfv  311 Mar 16 15:39 repodata.json.zst
-rw------- 1 wolfv 5.5K Mar 16 15:18 sdl_example-0.1.0-h60d57d3_0.conda
drwxr-xr-x 3 wolfv   96 Mar 16 15:39 shards

[wolfv]

I also tested with S3 and Anaconda and after the fixes in rattler (merged!) this is good :)