feat: allow user-preferred display ID when multiple IDs map to the same OID

[joeycarpinelli]

feat: allow user-preferred display ID when multiple IDs map to the same OID

Problem

display_id_for_oid in crates/git-forge/src/index.rs currently picks the first matching ID it finds via HashMap::iter().find_map():

pub(crate) fn display_id_for_oid(
    index: Option<&HashMap<String, String>>,
    oid: &str,
) -> Option<String> {
    index?.iter().find_map(|(key, val)| {
        if val == oid { Some(key.clone()) } else { None }
    })
}

When a forge issue is mirrored to multiple platforms (e.g. both tangled.sh and github.com), multiple index entries point to the same OID.
The function returns whichever key the HashMap iterator yields first, which is effectively random.
This means forge issue list and forge issue show may display github.com/42 one run and tangled.sh/7 the next, with no way for users to control it.

Examples

Given an index like:

Display ID	OID
tangled.sh/7	abc123
github.com/42	abc123

Running forge issue list might show:

tangled.sh/7   Fix the widget

or:

github.com/42  Fix the widget

Users who primarily work through one platform should be able to say "always prefer tangled.sh IDs when available."

Proposed solution

1. Add a preferred-platform (or similar) config key — e.g. forge config set preferred-platform tangled.sh.
   This tells forge which platform's display ID to prefer when multiple exist.

2. Update display_id_for_oid — first attempt to find a matching ID whose key starts with the preferred platform prefix.
   Fall back to the current behavior if no preference is set or no match exists for that platform.

   Sketch:

   pub(crate) fn display_id_for_oid(
       index: Option<&HashMap<String, String>>,
       oid: &str,
       preferred_platform: Option<&str>,
   ) -> Option<String> {
       let index = index?;
       let matches: Vec<_> = index
           .iter()
           .filter(|(_, v)| *v == oid)
           .map(|(k, _)| k.clone())
           .collect();
   
       if matches.is_empty() {
           return None;
       }
   
       if let Some(platform) = preferred_platform {
           if let Some(id) = matches.iter().find(|k| k.starts_with(platform)) {
               return Some(id.clone());
           }
       }
   
       Some(matches.into_iter().next().unwrap())
   }

3. Thread the preference through callers — the three call sites in
   crates/git-forge/src/issue.rs already have access to the repo; read
   the config value once and pass it down.