[Bug]: fork sets title by injecting into state.tags instead of a dedicated field

Bug Description

In local_conversation.py (line 876–881), when forking a conversation with a title, the title is set by injecting it into ConversationState.tags as tags["title"] = title. This conflates a display/metadata field with the user-defined tag map and means a forked conversation's tags will always contain a "title" key, which leaks into tag-based queries, observability span attributes, and any downstream consumer that iterates over tags.

# local_conversation.py:876
# Copy title via tags if provided
if title is not None:
    fork_conv._state.tags = {
        **fork_conv._state.tags,
        "title": title,
    }

This appears to be a workaround for ConversationState not having a dedicated title field — the agent server handles fork titles separately via StoredConversation.title.

Expected Behavior

A forked conversation's title is stored in a dedicated field and does not appear as a tag in the conversation's tag map.

Actual Behavior

Forking with a title injects {"title": "<value>"} into state.tags, polluting the tag map with a non-user-defined key. Reproduce with:

python -c "
from openhands.sdk.conversation.impl.local_conversation import LocalConversation
# fork a conversation with a title and inspect state.tags
# tags will contain {'title': 'My Fork'} alongside any user-defined tags
"

Or via the existing test suite:

pytest tests/ -k "fork" -v

Steps to Reproduce

1. Fork a conversation with a title
2. Inspect the forked conversation's state.tags
3. Observe that "title" appears as a key in the tag map alongside user-defined tags

Acceptance Criteria

• Forking with a title does not inject a "title" key into state.tags
• The fork title is stored in a dedicated field or passed through a mechanism that does not pollute the tag map
• Existing fork behavior in the agent server (StoredConversation.title) is unaffected

Additional Context

Discovered while investigating the boundary between ConversationState and StoredConversation (see #4810 and discussion in #proj-agent-server). ConversationState has no title field, which is why this workaround exists. The fix may require deciding where title lives in the standalone SDK context.

Start in local_conversation.py at lines 876–881 and inspect ConversationState and the fork path to determine where a title can live without entering tags. Run pytest tests/ -k "fork" -v, then verify that titled forks preserve user-defined tags and that agent-server StoredConversation.title behavior remains unaffected.