Allow create_time_entry to log time for another user (user_id)

[AndreaV-Lsi]

Problem / motivation

For agentic project management, we mirror a supplier's reported weekly hours into OpenProject to
track effort against a Time & Materials ceiling. Each entry should be attributed to the person who
did the work (e.g. a Tismo engineer) so their configured hourly rate drives the cost — not the
service/API account the server authenticates as. Today every entry is logged under the authenticated
API user, with no way to attribute it to someone else.

Feasibility spike (Core 17.5.1 — confirmed supported)

• POST /api/v3/time_entries exists; real entries carry a user link (observed one attributed to a
  different user than the API account).
• The time-entry schema reports user: writable=True required=True — so _links.user is settable
  on creation. When omitted, OpenProject defaults it to the authenticated user (today's behavior).
• Logging for another user requires the API user to hold the project permission "Log time for
  other users", and the target must be a project member; otherwise OpenProject returns 403/422.

Scope

• Tool (src/tools/time_entries.py, CreateTimeEntryInput): add
  user_id: Optional[int] = Field(None, gt=0, description="Log the time on behalf of this user (defaults to the authenticated user)"); pass into the data dict only when provided.
• Client (src/client.py, create_time_entry): when user_id is present, set
  payload["_links"]["user"] = {"href": f"/api/v3/users/{data['user_id']}"}.
• When user_id is omitted, behavior is unchanged. Mirror the optional-field pattern used for
  schedule_manually (tool field → data → client _links/payload). Keep the diff single-purpose.
• Document the "Log time for other users" permission dependency in the tool docstring; the existing
  try/except → format_error surfaces permission errors.

Acceptance criteria

• create_time_entry(..., user_id=<id>) creates an entry attributed to that user (verify via
  list_time_entries → _embedded.user).
• Omitting user_id logs under the authenticated user (unchanged).
• A missing-permission / non-member case returns a clear error, not a crash.
• Network-free unit test(s) (style of tests/test_update_readonly.py): _links.user set in the
  client payload when user_id given, absent when not.
• uv run black . && flake8 . clean on changed lines; uv run pytest passes. README updated.

Out of scope

• No unrelated reformatting. No change to the default log-as-authenticated-user behavior.
• update_time_entry re-attribution and bulk backfill (possible follow-up).

Notes

• Independent of PR-4 at the code level (adds a field to the wrapped CreateTimeEntryInput), but
  Cowork's JSON-string args need PR-4's CoercibleModel base — so for internal deployment it will be
  stacked on the fix branch; the single commit is cherry-pickable to main for a clean upstream PR.
• Suggested branch feature/time-entry-user; commits cite Refs #N.

Type: enhancement. Label: backlog.