chore(cli): remove obsolete tracevault push command

[softberries]

`push` was the legacy batch-at-end-of-session uploader. It POSTed a monolithic `PushTraceRequest` (commit metadata + diff + token totals + full transcript + session_data, all in one blob) to `/api/v1/orgs/{slug}/traces`.

That endpoint is no longer wired in the server — any POST to it gets a 404 today. The CLI command has been calling a missing endpoint and just hasn't been used enough for anyone to notice.

The replacement is already there

In the current pipeline the same data flows through two real-time paths installed by `init`:

• `tracevault stream` (Claude Code hook): per-event POST to `/stream` → populates `sessions`, `events`, `file_changes`, `transcript_chunks`
• `tracevault commit-push` (git post-commit hook): per-commit POST to `/commits` → populates `commits`, `commit_attributions`, `branch_tracking`

Both run automatically. The dashboard's GET endpoints under `/traces/*` (analytics, session detail, commit detail, timeline, attribution, branches) read from the same tables and keep working unchanged — only the write path changes.

Investigation summary

Before removing I verified there are no callers anywhere in the tree:

Consumer	Status
Server `POST /traces` route	Not registered — returns 404
`init` hooks (pre-push, post-commit, Claude)	Don't invoke `push`
GitHub Action (`action/action.yml`)	Only calls `verify`
Enterprise submodule	No references
Web dashboard	GETs only, never POSTs

Every field `PushTraceRequest` set (`total_tokens`, `cache_read_tokens`, `model_usage`, `compactions`, …) is now captured by `/stream`'s `final_stats` or `/commits`' `diff_data`. No data loss.

Changes

• Delete `crates/tracevault-cli/src/commands/push.rs` (852 lines)
• Remove `pub mod push` from `commands/mod.rs`
• Remove `Cli::Push` variant and handler from `main.rs`
• Remove `PushTraceRequest`, `PushTraceResponse`, and `ApiClient::push_trace` from `api_client.rs`
• README: drop `tracevault push` from the pre-push hook example, the commands table, and the authenticate section. Rewrite the latter to surface `flush` (retry queue) instead.
• `crates/tracevault-cli/README.md`: replace the quick-start `push` example with `check` and `flush`.

Paired website docs PR to follow.

Test plan

• `cargo fmt --all`
• `cargo clippy --all-targets -- -D warnings`
• `cargo test -p tracevault-cli` — all suites pass
• `tracevault --help` no longer lists `push`
• Manual: confirm an existing install continues to write sessions and commits via `stream` and `commit-push` with no `push` command in sight