vendors: map Freshdesk native export headers (Created time / Closed time / Resolved time)

[ursasi]

Closes #23.

What

Adds the native Freshdesk ticket-export headers to registry/vendor-rules.json
column_aliases:

Freshdesk native header	canonical column
Created time	opened_at
Closed time	closed_at
Resolved time	closed_at

Resolved time joins the existing Zendesk Solved at / Closed at mapping —
either status ends the conversation, so both land on closed_at.

Why

A Freshdesk standard ticket export labels its timestamps Created time /
Closed time / Resolved time (Freshworks support, How do I export my tickets
from Freshdesk?). After lower() those keep the space, so none of them matched
the alias table and a native export silently lost both timestamps — the
recount could not place a single line in time. Ticket ID already mapped via
the existing ticket id alias, so the id column was fine.

This is the same class of fix as commit ec0d06a (Zendesk space-form aliases):
data-only, no engine code, registry stays the single source of truth.

Files

• registry/vendor-rules.json — the three aliases (source of truth)
• healthcheck/am_healthcheck/vendor-rules.json — synced copy
• website/vendor-rules.js — regenerated via python3 scripts/gen_web_rules.py --build
• healthcheck/tests/test_vendors.py — two tests in TestNativeHeaderAliases

Verification

• python3 -m unittest discover -s healthcheck/tests → 454 tests, OK (was 452)
• python3 healthcheck/agentmeasure selftest → PASS
• python3 scripts/gen_web_rules.py --check → in sync
• python3 verify_vectors.py → ALL VECTORS PASS

Notes

• No new semantics invented: all three aliases map to columns that already exist.
• Agent / Resolution status were deliberately not aliased to
  human_agent_participated / issue_addressed. An assigned agent is a lead,
  not proof a human finished the conversation, and the status codes
  (2 Open / 3 Pending / 4 Resolved / 5 Closed) are not an issue-addressed
  judgement — those belong in a prepare_hints block, and prepare --vendor freshdesk would need a matching vendors.freshdesk billing block first, so
  it is left for a separate change.
• Minor pre-existing oddity: healthcheck/tests/test_vendors.py has
  if __name__ == "__main__": unittest.main() before TestNativeHeaderAliases,
  so running that file directly skips the class (discovery still finds it).
  Happy to move the guard to the end in this PR if you want.