[Bug]: get git repo identity reliably into Laminar traces

Goal

Laminar traces should reliably carry git repo/branch/commit identity for any conversation that ends up touching a repo — not just conversations where a repo was pre-selected at creation. Today they don't, for two compounding reasons below. Both need fixing to actually achieve this.

Problem 1: the metadata that IS collected gets silently discarded (mechanical bug)

In openhands/app_server/app_conversation/live_status_app_conversation_service.py, the observability_metadata dict built by _build_observability_context (added in OpenHands/OpenHands#14431) is immediately overwritten two lines later by a reassignment from _build_observability_metadata (added in OpenHands/OpenHands#15059) — at both conversation-request builders (the regular OpenHands path and the ACP path). Only observability_tags from the first call survives.

_build_start_conversation_request_for_user (~line 1801):

observability_metadata, observability_tags = self._build_observability_context(
    conversation_id,
    agent_kind='openhands',
    selected_repository=selected_repository,
    selected_branch=selected_branch,
    git_provider=git_provider,
)
...
observability_metadata = await self._build_observability_metadata(   # <-- clobbers the line above
    remote_workspace,
    project_dir,
    selected_repository,
    selected_branch,
    git_provider,
)
create_kwargs: dict[str, Any] = {'agent': agent, 'user_id': laminar_user_id}
if observability_metadata:
    create_kwargs['observability_metadata'] = observability_metadata
if observability_tags:
    create_kwargs['observability_tags'] = observability_tags

The identical pattern repeats in _build_acp_start_conversation_request (~line 2057-2097) for the ACP path.

_build_observability_context's metadata dict (app, conversation_id, agent_kind, repo_name, selected_branch, git_provider) is computed and then thrown away — it never reaches create_kwargs['observability_metadata']. Only its tags return value (app:openhands, agent_kind:..., repo:..., branch:..., git_provider:...) makes it through. The observability_metadata actually sent to Laminar ends up exclusively _build_observability_metadata's output (repo, branch, git_provider, commit), missing app/conversation_id/agent_kind.

Problem 2: even fixed, metadata only exists for repo-preselected conversations

_build_observability_metadata's repo/branch/git_provider/commit are all derived from selected_repository/selected_branch — the structured field passed at conversation creation (repo-picker flow). For a conversation started from scratch where the agent clones a repo itself in response to a chat message (selected_repository: null at creation — confirmed the dominant real-world flow via production trace analysis on infra#1454), this metadata stays empty for the whole conversation, even after the agent has cloned and is actively working in a repo. Fixing Problem 1 alone does not close this gap — most real conversations would still show no git identity in Laminar.

Suggested fix

1. Immediate: merge the two metadata dicts instead of one clobbering the other (e.g. metadata = {**context_metadata, **resolved_metadata}, or fold _build_observability_context's app/conversation_id/agent_kind fields into _build_observability_metadata and drop the duplicate call), at both call sites.
2. To actually meet the goal: populate/update git identity for conversations where the repo shows up after conversation start. Laminar metadata is currently only set once, at trace/request creation. Two directions worth weighing:
   • Reuse the live git-probe the workspace-archive endpoint already does (software-agent-sdk#3932, GET /api/file/archive's X-Archive-Repo-Remote/-Branch/-Head-Commit headers) to backfill/update trace metadata at archive time (pause/delete) — the only point today that already does a live probe outside of pre-agent setup.
   • Or hook into the agent's action/event stream to detect the first git clone/repo materialization and update trace metadata then — more general, more invasive.

Introduced by OpenHands/OpenHands#14431 landing on top of OpenHands/OpenHands#15059 (merged 2026-07-01 / 2026-07-09 respectively); Problem 1 didn't exist before either PR individually. Problem 2 predates both — OpenHands/OpenHands#15059 already had it, OpenHands/OpenHands#14431 didn't change it.

Start in openhands/app_server/app_conversation/live_status_app_conversation_service.py, reading _build_observability_context, _build_observability_metadata, _build_start_conversation_request_for_user, and _build_acp_start_conversation_request. Compare both request builders and inspect the GET /api/file/archive live git-probe headers. Done means Laminar retains the combined metadata and receives git identity for repositories discovered after conversation creation.