docs: apply analysis remediation edits

Applied 4 LOW-severity improvements from /speckit.analyze:

1. Clarified PNR format in FR-005 (TEST-{6 chars} not just 6 chars)
2. Added explicit FR-015 coverage note in tasks.md Phase 3
3. Documented infinite inventory design decision in T020
4. Connected documentation tasks to SC-006/SC-007 objectives

All changes are documentation clarifications, no functional impact.
Analysis showed 100% requirement coverage with zero blocking issues.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

specs/001-mock-gds-server/spec.md

@@ -118,7 +118,7 @@ A sales team demonstrates travel booking software to potential customers. They n
 
 - **FR-004**: System MUST support car rental search with parameters: pickup location (airport code), pickup date/time, dropoff location, dropoff date/time, returning available vehicle classes with daily rates, rental companies, and vehicle specifications
 
-- **FR-005**: System MUST support booking creation for any travel service (flight, hotel, car) with passenger/guest details, payment method placeholder (no actual processing), and return a unique 6-character alphanumeric PNR
+- **FR-005**: System MUST support booking creation for any travel service (flight, hotel, car) with passenger/guest details, payment method placeholder (no actual processing), and return a unique PNR in format TEST-{6-character alphanumeric} (e.g., TEST-A1B2C3)
 
 - **FR-006**: System MUST allow multiple service types (flight + hotel + car) to be combined under a single PNR, maintaining all segment details in one unified booking record