Improve PostHog usage auto-mapping and one-line install UX

[abrichr]

Summary

This PR upgrades homepage adoption signals and install UX with production-safe defaults.

1) Usage metrics from PostHog (server-side)

• Added exact-first event classification for:
  • demos
  • agent runs
  • GUI actions
• Added guarded pattern fallback only when exact mappings return no hits.
• Excluded low-signal/internal events from usage counts:
  • function_trace
  • get_events.started
  • get_events.completed
  • visualize.started
  • visualize.completed
• Added clearer source metadata in API payload.
• Fixed availability semantics: if PostHog is reachable/configured, usage source is treated as available even when counts are zero.

2) PostHog config ergonomics

• POSTHOG_PROJECT_ID now defaults to 68185 in API logic (still overridable by env).
• Added .env.example for required server-side keys.
• Added explicit handling for phc_ project token misuse:
  • returns actionable caveat that a personal API key (phx_) is required for reads.

3) Install UX

• Install section now shows a true one-liner first (per platform), with step-by-step fallback commands below.

Runtime verification

• npm run build passes.
• Deploy preview endpoint now returns configured PostHog source with no warnings:
  • /api/project-metrics reports source=posthog_event_definitions
• Current usage values are 0 because PostHog event-definition 30d volumes for mapped events are currently zero.

Notes

• (30d) means rolling last 30 days.
• This PR keeps env override support as fallback for manual counters.

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