feat(planning): make the interview settle window a setting with a longer default #7968

Refs: #6053

Agent Brief

Type: Feature
Summary: The interview server's quiet window and burst cap come from the settings layer, with a default quiet window of about 10 s so a sweep round of plain accepts wakes the agent once.

Current behavior:
Planning's server hard-codes QUIET_SECONDS = 3.0 and BURST_SECONDS = 12.0 and returns them from settle_window(). The audit run showed accepts 2-9 s apart (sweep seqs 42-51), so most plain accepts each cost one agent wake. Owner decision recorded on #6053 (2026-10-13): interim settings-layer QUIET_SECONDS of about 10 s.

Desired behavior:

• Two settings keys (for example settleQuiet and settleBurst, in seconds) resolve through the same settings layers as the existing keys (waitTimeout, leaseTimeout) and are documented in the interview session-config reference with their defaults.
• Defaults: quiet 10 s; burst raised with it so the burst cap stays above the quiet window (for example 30 s), and the burst never holds a reply past the watcher's wait deadline (the bridge's settle() already clamps to the deadline).
• Invalid values (non-numeric, zero or negative, quiet greater than burst) fall back to the defaults and do not stop the server.
• The settings panel shows the values read-only, like leaseTimeout.

Key interfaces:

• Planning's Hub.settle_window() return value: (quiet, burst) from settings instead of module constants.
• Settings shape: two new keys with defaults, documented in the interview session-config reference.

Acceptance criteria:

• scripts/run-test-file.sh plugins/planning/surface/test_server.py passes, including new cases: defaults return the new values; a configured value is returned; an invalid value falls back to the default.
• The session-config reference lists both keys with their defaults.
• The diff touches no file registered in scripts/shared-copies.txt.
• A planning changelog fragment is added and scripts/check-changelog-fragments.sh passes.

Out of scope:

• The bridge's own QUIET_SECONDS/BURST_SECONDS in lib/session-bridge/ (used by other apps' view bridges).
• Server-side auto-handling of plain accepts (sibling item).
• Measuring wake counts against a live session (not runnable unattended).

This was generated by AI during backlog shaping. Split from #6053; the owner approved filing it in an attended backlog-shaping session on 2026-10-18.