[refactor] Apply same detection cascade to documents_dir as output_dir (sister concern from #49)

[kiki830621]

Problem

Source: surfaced during /idd-plan #49 tangential sweep (Step 2.5)

plugins/che-apple-mail-mcp/commands/archive-mail.md line ~274 hardcodes documents_dir: correspondence/attachments as the default attachment routing destination. This is a parallel convention question to output_dir (the issue addressed in #49):

attachment_routing:
  ...
  data_dir: data/raw
  documents_dir: correspondence/attachments  # ← hardcoded default

After #49 lands, output_dir will use detection cascade (communications/email/ → correspondence/emails/ → default). But documents_dir will still assume correspondence/attachments/ — creating a layout inconsistency where emails go to communications/email/ but their attachments still flow to correspondence/attachments/.

Type

refactor / feature (parallel feature to #49)

Expected

documents_dir (and possibly data_dir) should benefit from the same detection-first treatment as output_dir:

• If cwd has communications/<channel>/ layout → documents_dir defaults to communications/email/attachments/ or communications/attachments/
• If cwd has correspondence/ layout → keep correspondence/attachments/ (legacy)
• If neither → current default

Actual

Hardcoded default. No detection. Inconsistent with #49's output_dir resolution after that lands.

Impact

• Users transitioning to communications/<channel>/ layout get split workspace: emails in communications/email/, attachments in correspondence/attachments/
• chchen_lab workspace would hit this immediately after archive-mail: support workspaces with nested communications layout and symlinked historical archives #49 lands
• Pure docs option: explicitly tell users to set both output_dir and documents_dir together

Priority

P3 — schedule. Becomes user-visible after #49 lands; before that, only theoretical.

Strategy sketch

Mirror #49's Tier B' detection cascade for documents_dir. Same probe-order policy (newest convention wins), same "explicit config wins" precedence. Likely a small follow-up PR (~50 LOC delta) once #49's detection helper is in place — can reuse the helper.