Make telemetry reliable across interrupts and headless installs

Three reliability gaps surfaced while auditing why PostHog numbers were
untrustworthy:

1. Interrupted commands lost their events. capture() is fire-and-forget and the
   only flush guarantee lived in a finally block, which SIGINT/SIGTERM skip — so
   Ctrl-C'ing a long ingest or an MCP client killing 'ktx mcp stdio' dropped the
   command event and any queued events. Add SIGINT/SIGTERM handlers (real-process
   entry only; never under test/programmatic io) that mark the active command
   span aborted, emit it, drain the emitter, then exit. Idempotent with the
   normal finally path via the single-consume command span.

2. Headless-first installs were invisible. loadTelemetryIdentity refused to mint
   an installId unless stdout was a TTY, so a machine whose first run was an
   IDE-launched MCP server or a script emitted nothing, ever. Mint on first run
   regardless of surface (still honoring CI/DO_NOT_TRACK/KTX_TELEMETRY_DISABLED),
   writing the one-time notice to stderr — safe under the MCP stdio protocol,
   which reserves stdout. Drop the now-unused stdoutIsTTY option.

3. No guard against silent emit regressions (the 0.7.0 scan_completed blackout).
   Add tests: the shared executePublicIngestTarget chokepoint emits exactly one
   ingest_completed on success and on the preflight-failure branch, and a
   database target invokes the scan that emits scan_completed; plus coverage for
   the aborted-flush helper.

Identity is unchanged otherwise: every event still attributes to the installId
in ~/.ktx/telemetry.json. No event/field changes, so Node<->Python schema parity
is untouched. Docs updated to reflect first-run-on-any-surface activation.

docs-site/content/docs/community/telemetry.mdx

@@ -6,11 +6,10 @@ description: Understand what usage telemetry ktx collects and how to opt out.
 **ktx** collects aggregated usage telemetry so maintainers can see
 which commands work, where setup fails, and which parts of the data-agent
 workflow need improvement. Telemetry is opt-out: it turns on the first time you
-run **ktx** in an interactive terminal, which prints a one-time notice. From
-then on the same install also reports background activity that has no terminal
-of its own, such as the local MCP server your agent calls. It stays disabled in
-CI, whenever an opt-out is set, and until that first interactive run has shown
-the notice.
+run **ktx** in any way — an interactive command, a script, or an
+agent-launched MCP server — and prints a one-time notice (to the terminal when
+there is one, otherwise to standard error). It stays disabled in CI and whenever
+an opt-out is set.
 
 ## Opt out