feat(slack): briefing continuity lead-line + Slack-renderable link refinement

[jordanrburger]

Part of the de-personalized Patterns batch — upstreaming accumulated instance rules into the engine phases (Phase-1 work for the connector-based migration, #156). Follows the method from #157.

What

Two rules added to phases/connectors/slack.md notification slot (assembles into SKILL only — connector phase, requires: slack, mode: [consolidation, briefing]):

• Lead the briefing DM with a continuity line (Pattern feature: scoutctl bootstrap auto — unified install/upgrade entrypoint #26) — a briefing wrap-DM must open with a one-line "Today's Focus" continuity statement connecting recent work to what matters today, before the counts and lists. Counts tell the user how much; the continuity line tells them what to care about first. (Updates the briefing-notification example to show the lead line, plus a rule.)
• Slack-renderable link refinement (Pattern perf: three quick wins — launcher cache, schedule.yaml mtime cache, watch.sh opt-in polling (closes #81, #82, #83) #91) — refines the existing "link the action-items file" rule: Slack does not auto-linkify custom URL schemes (obsidian://, file://), so a bare URI renders as inert text. Wrap it in Slack mrkdwn angle-bracket form <url|label>; GFM [label](url) also fails for custom schemes. The principle generalizes — a clickable-link fix is only delivered when the link renders in the destination surface's markup dialect.

Verification

• Assembly checked via _assemble(cfg, kind): both rules land in SKILL only, not DREAMING/RESEARCH (mode: [consolidation, briefing]).
• Diff grepped clean of names / employer / customers / Slack IDs / repo-PR-Linear IDs / user paths (the obsidian:///file:// examples use the existing {{INSTANCE_NAME}} / {{SCOUT_DIR}} placeholders). A semantic-layer token visible near the diff is a pre-existing context line in the unchanged shortcode-legibility rule, not part of this change.

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