fix: correct zip root path and add idempotent build.prop append

- Fix ZIPROOT path from 3 to 4 levels up (../../../../)

- Add path validation before install

- Replace simple cat append with key-based dedup for build.prop (skips existing keys to prevent duplication on re-flash)

- Update AGENTS.md and README.md to reflect the changes

README.md

@@ -76,9 +76,10 @@ During flash, TWRP will show:
 
 ## Notes
 
-- The `build.prop` entries are **appended, not replaced**. Re-flashing
-  the zip multiple times will duplicate the entries — restore from
-  `/system/build.prop.bak` if needed, or inspect the file first.
+- The `build.prop` entries are **appended with dedup** — each property key
+  (`key=value`) is only added if the key does not already exist in
+  `/system/build.prop`. Re-flashing the zip multiple times is safe and
+  will **not** duplicate entries.
 - The `sensor_config.xml` covers four camera sensor combinations:
   rear (`hi846`, `gc8034_rear`) and front (`hi846_front`, `gc8034`),
   each with OTP EEPROM tuning parameters.