## Issue

Closes #16875 

Clarify how Coder authentication works with Git providers, particularly
the order of authentication methods used.

## Changes Made

I've updated the External Authentication documentation to:

1. Clarify that Coder first attempts to use external auth provider
tokens when available, and only defaults to SSH authentication if no
tokens are available
2. Add more detailed explanations about both authentication methods
3. Improve the description of how the `coder gitssh` command works with
existing and Coder-generated SSH keys

## Verification

Claude verified that this accurately describes the behavior of the
codebase by reviewing the `gitssh.go` implementation, which shows how
Coder handles SSH authentication as a fallback when external auth is not
available.


[preview](https://coder.com/docs/@16875-git-workspace-auth/admin/external-auth)

<sub>🤖 Generated with https://claude.ai/code</sub>

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Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: EdwardAngert <17991901+EdwardAngert@users.noreply.github.com>
Co-authored-by: Ben Potter <me@bpmct.net>
Co-authored-by: M Atif Ali <atif@coder.com>
Co-authored-by: Bruno Quaresma <bruno@coder.com>
Co-authored-by: Kyle Carberry <kyle@coder.com>
Co-authored-by: Cian Johnston <cian@coder.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Jon Ayers <jon@coder.com>
Co-authored-by: Hugo Dutka <hugo@coder.com>
Co-authored-by: Ethan <39577870+ethanndickson@users.noreply.github.com>
Co-authored-by: Michael Smith <throwawayclover@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Sas Swart <sas.swart.cdk@gmail.com>