build(telemetry): link Sentry conditionally and document privacy model

[cameroncooke]

Summary

Establishes the opt-in build gate for Sentry telemetry and the privacy documentation. Adds no runtime behavior on its own — it only makes telemetry possible to compile in, and inert by default.

Stack: 1/10 · base main · Refs GH-183

What this changes

• Package.swift: reads REPOPROMPT_ENABLE_SENTRY at manifest-eval time; only when =1 does it add the sentry-cocoa dependency and define REPOPROMPT_SENTRY_ENABLED. Pins sentry-cocoa 9.17.1 (Package.resolved).
• AppBundle/Info.plist.template: adds an empty RepoPromptSentryDSN slot.
• ThirdPartyLicenses/* + swiftpm_notice_guardrails.sh: registers sentry-cocoa's MIT license so the notice guardrail passes.
• docs/privacy/telemetry.md: the human-readable privacy contract.

Business rules

• Telemetry is OFF by default. DEBUG, self-compiled, locally-signed, and ad-hoc builds never link the SDK; the #if REPOPROMPT_SENTRY_ENABLED blocks in later PRs compile to nothing.
• Only official Developer ID release builds set REPOPROMPT_ENABLE_SENTRY=1.
• The DSN slot ships empty; it is populated only by the release pipeline (PR 10).

Key decisions

• Conditional dependency, not always-link-then-gate-at-runtime. This makes the SDK structurally absent from any non-telemetry build — privacy by construction, not by configuration.
• Hoisted the dependency/settings arrays into mutable vars so the if sentryEnabled { … } block is the single, auditable opt-in point.

Test plan

• swift build (no flag) resolves and builds with the SDK absent.
• REPOPROMPT_ENABLE_SENTRY=1 swift build links sentry-cocoa from cache. ✅ verified (rc=0).

[cameroncooke]

• build(telemetry): generate and upload dSYMs in the release pipeline #332
• feat(telemetry): instrument Codex runtime events and session persistence #331
• feat(telemetry): instrument Agent Mode run lifecycle #330
• feat(telemetry): instrument workspace actions #329
• feat(telemetry): instrument file, prompt, selection and apply-edits tools #328
• feat(telemetry): instrument context builder tool runs #327
• feat(telemetry): instrument MCP tool dispatch and bootstrap admission #326
• feat(telemetry): start telemetry and trace app launch #325
• feat(telemetry): add typed Sentry telemetry model and bootstrap #324
• build(telemetry): link Sentry conditionally and document privacy model #323 👈 (View in Graphite)
• main

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