[BUG] apm does not respect the default branch

[srid]

Describe the bug
When a repo uses anything but main as the branch name, apm install fails because it is hardcoded to use master.

To Reproduce

srid on pureintent AI on  main [!?] via λ 
❯ cat apm.yml 
name: AI
version: 1.0.0
type: hybrid

dependencies:
  apm:
    - srid/agency

srid on pureintent AI on  main [!?] via λ 
❯ uvx --from git+https://github.com/microsoft/apm apm install
[>] Installing dependencies from apm.yml...

-- Diagnostics --
  [x] 1 package failed:
    +- srid/agency -- Failed to install srid/agency: Failed to clone repository srid/agency 
using all available methods. Please check repository access permissions and authentication 
setup. Last error: Cmd('git') failed due to: exit code(128)
  cmdline: git clone -v --depth=1 --branch=main -- https://github.com/srid/agency 
/tmp/tmpvtngir8z
  stderr: 'Cloning into '/tmp/tmpvtngir8z'...
POST git-upload-pack (338 bytes)
fatal: Remote branch main not found in upstream origin
'

[x] Installation failed with 1 error(s).

srid on pureintent AI on  main [!?] via λ 
❯ 

Expected behavior

It succeeds.

Environment (please complete the following information):

• OS: macOS
• Python Version: uvx 0.11.0 (x86_64-unknown-linux-gnu)
• APM Version: from git (see uvx command above)

[srid]

LLM analysis

APM hardcodes "main" as the default branch in ~7 places across two files:

• src/apm_cli/models/dependency/types.py:50 — parse_git_reference returns ("branch",
  "main") when no #ref is specified
• src/apm_cli/deps/github_downloader.py — six more ref = dep_ref.reference or "main"
  fallbacks

It does not query the remote's actual default branch (no git ls-remote, no GitHub API
call). There's a reactive fallback — if main 404s it tries master — but that only works
for some code paths (GitHub API downloads). The git clone --branch=main path you hit
doesn't have that fallback, so it just fails.

No open issues about this on microsoft/apm. Might be worth filing one — the fix would be
to either omit --branch and let git use the remote HEAD, or do a git ls-remote --symref to
detect it.