fix(recon): resolve Classroom 50 roster columns in discover_repos

[agiacalone]

Problem

A Classroom 50 roster.csv and a legacy GitHub Classroom roster collide on the github_id column, and they mean different things by it:

Column	Legacy GitHub Classroom	Classroom 50
github_id	the login (bruce-wayne)	the immutable numeric id (1548364)
username	absent	the login
name	name / canonical_name	split across first_name + last_name

Because discover_repos() read github_id before anything else, feeding it a C50 roster produced three failures at once — none of them loud:

• repo names built from the numeric id: <prefix>-1548364 instead of <prefix>-agiacalone
• every row with an unresolved numeric id silently dropped from the population
• student names blank

Reproduced against the live c50-throwaway-tst0 roster: 1 of 11 rows resolved, and the one that did was named wrong.

Fix

Identifier precedence is now github_username → username → github_id, with a first_name + last_name fallback for the student name.

github_id stays last in the chain, so legacy behavior is unchanged whenever username is absent, and the explicit github_username override still outranks both.

Verification

• 11/11 rows resolve against the live C50 roster; agiacalone's repo name matches the real repo on GitHub exactly
• legacy roster shape unchanged (covered by a new back-compat test)
• full suite: 411 passed, 1 skipped
• the three new C50 tests were confirmed RED against the unpatched module before landing — a green test proves nothing until it has gone red

Not in this PR

repo_prefix carries two incompatible join conventions: recon_discover and triage_report require a trailing -, while feedback_deliver builds f"{org}/{repo_prefix}-{gid}" and requires none. It isn't broken today — those load from separate manifest files with separate values — but the C50 migration is exactly when a prefix gets copied between them and yields ...test--agiacalone. Left alone here to keep this fix clean and narrowly tested.