docs: add KNOWN-ISSUES.md and rewrite PowerAssertion top doc

PowerAssertion file-level doc previously claimed Linux logind and
Windows SetThreadExecutionState implementations in present tense.
Both are no-ops; the macOS path compiles but is unverified on
Apple Silicon (RB-08). Rewrite top doc to state present vs
planned posture and reference KNOWN-ISSUES.md.

Surfaced as tracked limitations:
- KI-01: macOS App Nap guard pending Apple Silicon verification
- KI-02: Linux power assertion is a no-op
- KI-03: Windows power assertion is a no-op
- KI-04: magnotia-cloud-providers crate not user-exposed in v0.1
  (in-memory keystore needs OS keychain before any save-key UX)

README links to KNOWN-ISSUES.md from Status, Platform support
table, and Project documentation. Platform support table notes
adjusted per OS to reflect actual idle-inhibit posture.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

KNOWN-ISSUES.md

@@ -0,0 +1,51 @@
+# Known issues
+
+Tracked limitations and partial implementations in the current codebase. Each entry points at the code that owns the gap so future work can find it.
+
+## Power management
+
+### KI-01 — macOS App Nap guard pending Apple Silicon runtime verification
+
+**Status:** macOS code path compiles and the registry tests pass, but runtime behaviour against actual OS idle-throttling has not been confirmed on Apple Silicon hardware. Also tracked internally as **RB-08**.
+
+**Source:** [`src-tauri/src/commands/power.rs:73`](src-tauri/src/commands/power.rs#L73) (`PowerAssertion::begin`), [`src-tauri/src/commands/power.rs:140`](src-tauri/src/commands/power.rs#L140) (`objc_bridge`).
+
+**Impact:** Long live-dictation sessions on macOS may still be slowed or paused by the OS until verified end-to-end on hardware.
+
+**Workaround:** Keep the app window focused, or move the cursor periodically. For testing, App Nap can be disabled globally with `defaults write NSGlobalDomain NSAppSleepDisabled -bool YES` (revert with `-bool NO`).
+
+### KI-02 — Linux power assertion is a no-op
+
+**Status:** `PowerAssertion::begin` does nothing on Linux. The planned implementation (systemd-logind / GNOME idle inhibitor via `org.freedesktop.login1.Inhibit`) is described in the file-level doc but not wired up.
+
+**Source:** [`src-tauri/src/commands/power.rs`](src-tauri/src/commands/power.rs).
+
+**Impact:** Long sessions can be paused by the compositor's idle hooks (screen lock, suspend timers) on KDE, GNOME, Hyprland, Sway, etc.
+
+**Workaround:** Raise the system idle / screen-lock timeout while dictating, or wrap launch with `systemd-inhibit --what=idle:sleep:handle-lid-switch ./run.sh`.
+
+### KI-03 — Windows power assertion is a no-op
+
+**Status:** `PowerAssertion::begin` does nothing on Windows. The planned implementation (`SetThreadExecutionState(ES_CONTINUOUS | ES_SYSTEM_REQUIRED | ES_AWAYMODE_REQUIRED)` on begin, `ES_CONTINUOUS` alone on end) is described in the file-level doc but not wired up.
+
+**Source:** [`src-tauri/src/commands/power.rs`](src-tauri/src/commands/power.rs).
+
+**Impact:** Long sessions can be paused by Windows sleep policies.
+
+**Workaround:** Set the active power plan's sleep to "Never" while dictating.
+
+## Cloud providers
+
+### KI-04 — `magnotia-cloud-providers` crate is not user-exposed
+
+**Status:** The crate is a declared workspace dependency in [`src-tauri/Cargo.toml:36`](src-tauri/Cargo.toml#L36) and compiles into the binary, but no Tauri command, page, or settings field invokes `store_api_key` / `retrieve_api_key`. There is no UI to enter or store a cloud API key in the current build.
+
+**Source:** [`crates/cloud-providers/src/keystore.rs`](crates/cloud-providers/src/keystore.rs). The `TODO` in the doc comment captures the planned migration to OS-native credential storage (the `keyring` crate or equivalent) before the feature is wired up.
+
+**Impact:** None for end users today (no exposed surface). When the feature is wired post-launch, cross-restart persistence must land before any "save key" UX ships, otherwise saved keys reset on every restart.
+
+**Workaround:** N/A.
+
+## How to add an entry
+
+When shipping a partial implementation or known limitation, add a `KI-NN` entry here with the four standard fields: **Status**, **Source** (file:line), **Impact**, **Workaround**. Link from the affected module's doc comment back to this file by ID.