docs: in-app timeline user guide — explain scheduling modes, featureMode vs scheduleMode, and resource levelling

[NickMonrad]

Overview

The timeline is the most complex screen in the app and users need guidance on how to interpret and configure it correctly. This issue tracks an in-app help guide (and/or a static /help/timeline page) covering the key concepts.

Key concepts to cover

Epic scheduling modes

• scheduleMode: sequential (default) — this epic waits for all features of the previous epic to finish before starting
• scheduleMode: parallel — this epic starts at week 0 alongside other parallel epics; it is NOT chained after earlier epics
• ⚠️ Common misconception: scheduleMode: parallel does NOT make features within the epic run in parallel — it only controls how the epic relates to other epics

Feature scheduling modes (featureMode)

• featureMode: sequential (default) — features within the epic run one after another, sorted by their order number
• featureMode: parallel — features within the epic are all scheduled as early as resources allow; they run concurrently when there is capacity
• To get features truly running in parallel, BOTH modes must be set appropriately

Resource levelling

• The scheduler respects named resource capacity (hoursPerDay × count) and will not overallocate a resource type
• Features are queued using a priority heap (lowest epicOrder + featureOrder wins) — features at the same order number are tie-broken arbitrarily
• A feature cannot start until its resource is available AND all its predecessors have finished

Typical scenario: PoC running alongside delivery

1. Set the PoC epic's scheduleMode → parallel (so it doesn't wait for the delivery epic)
2. Set the PoC epic's featureMode → parallel (so PoC features run concurrently within the epic)
3. Result: all PoC features compete for resources from week 0 alongside delivery features

Gaps in the timeline

• A gap (idle week) for a resource usually means it is occupied by a different feature from a different epic running concurrently
• Use resource utilisation tooltips (Timeline: show resource utilisation tooltip on feature bars #97) to diagnose contention

Acceptance criteria

• A help panel or tooltip accessible from the Timeline page explains scheduleMode and featureMode
• The explanations include the common misconception about scheduleMode: parallel not affecting intra-epic sequencing
• Optionally: a static /help/timeline page that can be linked from a footer FAQ (feat: support contact form and footer links (support, FAQ, how-to) #75)