[Story]: Define a vendor-neutral AI code-suggestion acceptance schema

## Summary
Define a canonical, vendor-neutral schema for AI code-suggestion acceptance events: `shown / accepted / rejected / edited-before-accept / retained-after-N-days`. Every downstream connector and dashboard depends on this schema existing first. Must explicitly separate ghost-text tab-acceptance (Copilot/Cursor inline completions) from agent-mode diff-approval (Claude Code, Junie-style multi-file proposals) — these are not comparable events and conflating them would corrupt every aggregate metric downstream.

## Intent / Source of truth
Epic finding: "There is no standard to lean on — we must define the schema" (OTel GenAI semantic conventions, `Status: Development`, define token usage/duration/spans but nothing for acceptance). 2026-08-13 backlog consolidation.

## Scope
- [ ] Canonical event schema (fields, enums, timestamps) covering `shown/accepted/rejected/edited-before-accept/retained-after-N-days`
- [ ] Explicit separation of ghost-text tab-acceptance vs agent-mode diff-approval as distinct event subtypes with documented denominator implications
- [ ] Per-vendor denominator mapping doc (what each vendor actually counts as the numerator/denominator for "acceptance rate")
- [ ] Forward-compatibility note: how this schema would remap onto OTel GenAI semantic conventions if/when that spec ever standardizes acceptance

## Out of scope
- Building any connector against this schema (that's stories 2-5)
- Retroactively normalizing historical data collected before this schema existed

## Verification
Schema document reviewed and merged; at least one downstream connector story (2-5) can cite concrete field mappings into this schema without needing schema changes.

## Risk assessment
Getting this wrong early is expensive — every connector built against it inherits the mistake. Prioritize correctness over speed; get the ghost-text/agent-mode separation right before any connector ships.

## AI Usage Declaration
Drafted with AI assistance during the 2026-08-13 cross-repo backlog consolidation. A human owns intent, verification and consequences; acceptance criteria must be verified against the live system before this is closed.

No files or tests are named. Start with the OTel GenAI semantic conventions and the vendor denominator mappings identified in the issue, then define the canonical fields, enums, timestamps, and distinct ghost-text and agent-mode subtypes. Done means the schema document is reviewed and merged, and at least one connector story can cite concrete field mappings without requiring schema changes.

Scritto dal modello di indicizzazione a partire dal testo della issue.