fix(dav): upsert upload matches server-converted ext (xls→xlsx) (#82)

OnlyOffice converts legacy binary Office uploads (.xls/.doc/.ppt) into
OOXML (.xlsx/.docx/.pptx) on the server. UploadToFolderReplacing matched
by the exact stem|ext via FindFilesByDedupKey, so a repeated
`oo dav upload FOLDER f.xls --replace` never found the stored f.xlsx and
appended a second file (live: ids 3799+3887, 3800+3888).

- EquivalentUploadExt / FindFilesByStemExt: match by stem with a
  legacy↔OOXML extension equivalence, so .xls finds the saved .xlsx.
- planUploadReplacement: pick the surviving file and the redundant
  duplicate ids for a replacing upload.
- UploadToFolderReplacing updates the existing file in place (UpdateFile,
  stable id, no delete window), removes extra duplicates, and falls back
  to conversion-aware delete + upload when the portal rejects the update.
- AssertNoFileConflict (--no-replace) uses the same conversion-aware
  matching so a .xls upload conflicts with an existing .xlsx.
- Offline tests cover the matcher, the plan and the repeated-.xls
  regression; pdf/xlsx behaviour unchanged.

AGENTS.md

@@ -30,7 +30,7 @@ Canonical Go client for OnlyOffice Workspace (Projects + Calendar + CRM) and the
 - Prefer `ResponseObject` / `postFormObject` / `putFormObject` / `deleteObject` over hand-rolled `json.Unmarshal(responseField(...))` blocks — they exist for DRY, use them.
 - Domain split is by file, **not** by subpackage. Don't introduce `internal/` or `pkg/*` subpackages inside the library — it flattens the `*Client` call surface for a reason.
 - CLI commands follow **subject → verb** structure (`oo <subject> <verb>`), never `oo <verb>-<subject>`. Add new commands to the existing subject file if one fits; create a new `cmd/oo/<subject>.go` for a genuinely new domain. The subject→verb tree in `cmd/oo/main.go` and the README table are documentation — update them with the code.
-- **Documents for agents:** prefer Markdown in git; OnlyOffice UI is weak for `.md`/`.txt`. Use `oo docs put-md` (md→docx) and `oo docs put-txt` (txt→docx, preserves line breaks). All upload paths default to **upsert** by `stem|ext` (`--replace`, default true); `--no-replace` fails on conflict; `--allow-duplicate` opts into raw OO append. `oo projects files dedupe PROJECT_ID` reports/removes duplicate stem|ext copies (`--apply`, `--cross`; includes project root folder).
+- **Documents for agents:** prefer Markdown in git; OnlyOffice UI is weak for `.md`/`.txt`. Use `oo docs put-md` (md→docx) and `oo docs put-txt` (txt→docx, preserves line breaks). All upload paths default to **upsert** by `stem` with the extension matched conversion-aware (legacy `.xls/.doc/.ppt` ↔ OOXML `.xlsx/.docx/.pptx`, since OnlyOffice converts them on upload), so a repeated `.xls` upload updates the saved `.xlsx` instead of appending a duplicate (`--replace`, default true); `--no-replace` fails on conflict; `--allow-duplicate` opts into raw OO append. `oo projects files dedupe PROJECT_ID` reports/removes duplicate stem|ext copies (`--apply`, `--cross`; includes project root folder).
 - Every table output goes through `printTable(headers, rows)`; every single-object through `printObject(v)`. Do not `fmt.Println` rows ad-hoc or the `--output json` flag breaks for that command.
 - No secrets in the repo; use `.env` (gitignored). Commit `.env.example` only.
 - **No host/client specifics in the tree.** Endpoints, IPs/ports, client mail