chore(sync): bump upstream-version.txt to v3.17.4 (#63)

* chore(sync): bump upstream-version.txt to v3.17.4

The file tracked v3.17.0 but the conservative-sync window for
v3.17.1 -> v3.17.4 was already merged in earlier work:

- cb15a7b6 chore(sync): merge upstream v3.17.3
- 630a11d6 release: v3.17.4
- c9350c67 release: v3.17.4

Bumping upstream-version.txt to v3.17.4 aligns the tracked state
with the merged state. The upstream-tag-watcher workflow compares
this file against upstream/code-yeongyu/oh-my-openagent tags; with
upstream currently at v3.17.4 as well, the watcher will now report
'Upstream unchanged' until a new upstream stable tag lands, instead
of repeatedly attempting to re-analyze v3.17.0 -> v3.17.4.

This is the outstanding drift flagged in NEXT_SESSION_HANDOFF.md
section E item 1.

* docs(fork-deviations): align snapshot reference with v3.17.4 bump

Addresses Devin review note on PR #63: the snapshot paragraph (line
11-12) named v3.17.0 as the last sync target, but this PR bumps
upstream-version.txt to v3.17.4. Updated to v3.17.4 with a note
explaining the v3.17.0 → v3.17.4 transition so readers of the
document understand the ordering.