Simplify PR dashboard state loading

[trask]

Misc refactoring. I split this out of a bigger (to come) PR addressing #40.

Replace transient in-band dashboard and notification state markers with explicit missing-state handling.

Targeted dashboard refreshes now skip cleanly without accepted cached state, notification processing skips without dashboard state, and Slack notification calculations operate on per-PR notification records with delivery errors returned separately.

Also a good bit of renaming to make it clearer what is going on in some places.

[Copilot]

Pull request overview

This PR refactors state loading in the PR dashboard tooling, replacing the transient in-band _loaded_from_dashboard (and _notification_errors) markers with explicit missing-state handling via None returns. The result is a clearer contract: a targeted single-PR refresh now skips cleanly when there is no accepted cached dashboard state (instead of falling back to a full rebuild), notification processing skips when dashboard state is absent, and Slack notification computation operates on per-PR notification records with delivery errors returned as a separate value.

Changes:

• load_state_file and cache loaders now return None for missing/invalid state, and legacy _-prefixed markers are stripped on load.
• Notification helpers are reworked to per-PR records (load_notifications/save_notifications/union_merge_notifications), and next_notifications returns (notifications, delivery_errors).
• Single-PR refreshes skip when no accepted dashboard state exists; compute_pr_results receives dashboard_state as an explicit argument.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
.github/scripts/pull-request-dashboard/state.py	Return None for missing state, strip legacy markers, add per-PR notification load/save/merge helpers
.github/scripts/pull-request-dashboard/notify_slack.py	Skip on missing dashboard state, merge retry snapshot via last_notifications, rename retry-snapshot/error paths
.github/scripts/pull-request-dashboard/notifications.py	Rename to next_notifications, return errors separately, gate initial notifications on has_last_notifications
.github/scripts/pull-request-dashboard/dashboard.py	Pass dashboard_state into compute_pr_results, skip targeted refresh on cache miss, handle None in reconciliation

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