docs(capture): document capture_mode, capture_compression, and changeset

Adds the Sampo changeset for the opt-in capture_mode (v1 ingestion
protocol) and capture_compression, plus an AGENTS.md section mapping
capture_mode/capture_compression and their env vars to the modules and
routing that implement them, the v1 invariants to preserve, and the
sync_mode blocking-retry behavior.

User-facing usage stays in the official docs per the README convention.

Author: eli-r-ph

.sampo/changesets/capture-v1-mode.md

@@ -0,0 +1,7 @@
+---
+pypi/posthog: minor
+---
+
+Add an opt-in `capture_mode` for the Capture V1 ingestion protocol (`POST /i/v1/analytics/events`). Set `capture_mode="v1"` on the client (or the `POSTHOG_CAPTURE_MODE=v1` environment variable) to use Bearer auth, per-event results, and partial retry. Defaults to `"v0"` (the legacy `/batch/` endpoint), so existing setups are unaffected.
+
+When using `capture_mode="v1"`, request bodies can be compressed via `capture_compression` (or `POSTHOG_CAPTURE_COMPRESSION`): `"gzip"`, `"deflate"`, or `"none"` (default). The legacy `gzip=True` flag is honored as a fallback.

AGENTS.md

@@ -9,6 +9,26 @@ Guidance for coding agents working in `posthog-python`.
 - The project uses `uv` for local development. See `CONTRIBUTING.md` for setup.
 - Keep edits targeted and follow existing patterns. Prefer adding or updating tests near the behavior you change.
 
+## Capture protocol (`capture_mode`)
+
+The client supports two ingestion wire protocols, selected by `capture_mode` (precedence: explicit `Client(capture_mode=...)` kwarg > `POSTHOG_CAPTURE_MODE` env var > default).
+
+- `"v0"` (default) — legacy `POST /batch/`. Upgrades stay transparent; existing callers are unaffected.
+- `"v1"` — `POST /i/v1/analytics/events`: Bearer auth, a typed event `options` object, per-event results, and partial retry.
+
+v1 request bodies can additionally be compressed via `capture_compression` (precedence: explicit `Client(capture_compression=...)` kwarg > `POSTHOG_CAPTURE_COMPRESSION` env var > the legacy `gzip` flag > none). Supported values are `"none"`, `"gzip"`, and `"deflate"` (zlib-wrapped, RFC 1950, to match the server's decoder and the Go/Rust SDKs). v0 keeps using its own `gzip` flag; `capture_compression` is v1-only.
+
+Where the pieces live:
+
+- `posthog/capture_mode.py` — the `CaptureMode` enum and `resolve_capture_mode()` precedence logic.
+- `posthog/capture_compression.py` — the `CaptureCompression` enum and `resolve_capture_compression()` precedence logic (with `gzip` fallback).
+- `posthog/capture_v1.py` — pure transforms (`to_v1_event`, `build_v1_batch_body`) and transport (`post_v1`, `_compress_v1`, `parse_v1_response`, `send_v1_batch`, `CaptureV1Error`).
+- Routing: `Consumer._send_analytics` (async) and `Client._enqueue` (sync) pick the analytics submitter by `capture_mode`. The dedicated `$ai_*` endpoint has no v1 form and always uses the legacy submitter.
+
+v1-specific behavior to preserve when editing: sentinel `$`-properties are lifted into `options` (coerced to native JSON types or omitted — a wrong type 400s the whole batch); top-level `$set`/`$set_once` are relocated into `properties`; only events the server tags `retry` are resent (stable `PostHog-Request-Id`/`created_at`, incrementing `PostHog-Attempt`); `429` is terminal.
+
+Retry blocking matches v0: in the default async mode retries happen on the background consumer thread, but with `sync_mode=True` the partial-retry loop (including its backoff sleeps) runs inline on the calling thread, so a slow/erroring endpoint blocks the caller until retries are exhausted.
+
 ## Validation
 
 Useful checks: