create-implementation-plan: require unique identifiers (#1989) (#2142)

* create-implementation-plan: require unique identifiers (#1989)

The skill template tells the agent to use REQ-, TASK-, GOAL-, and
similar prefixed identifiers, but never says they have to be unique
or how to check. @basilevs reported plans coming back with duplicate
TASK IDs and proposed three POSIX one-liners that catch the two real
collision modes (table rows and bullet declarations) plus a broad
diagnostic scan.

Document the uniqueness rule under the existing Template Validation
Rules, then add a new "Identifier Uniqueness Check" section with all
three bash commands and instructions on which must come back empty
before the plan is finalized.

DEP-* references intentionally allowed in multiple sections per the
reporter's note.

Closes #1989.

* codespell: ignore GUD identifier prefix (#1989)

Upstream skills/create-implementation-plan/SKILL.md already uses
GUD-001 in the template body. Codespell currently slips past it on
word-boundary, but the regex alternation (GUD|RISK|...) added in the
previous commit on this branch makes codespell flag it as a misspelling
of GOOD.

GUD is the documented "Guideline" identifier prefix alongside REQ,
SEC, CON, PAT, etc. Add it to the ignore-words-list, matching the
pattern every other technical-token exemption in .codespellrc uses.

* create-implementation-plan: clarify declaration vs reference (#1989 review)

basilevs flagged that calling out DEP-* specifically was misleading,
because any identifier can appear as a reference. A TASK body can
cite a REQ, one TASK can cite another, and so on. The original
phrasing made it sound like DEP-* was the only prefix allowed to
recur.

Rewrite the rule to lead with "uniquely declared":
- Define declaration as the leading bullet/cell ID (e.g., the table
  row in Implementation Phase N, or '- **REQ-001**:').
- Say explicitly that references elsewhere in the plan are expected
  and not collisions, with concrete examples (TASK citing REQ,
  TASK citing TASK, Dependencies pointing at a DEP declared upstream).
- Tighten the check intro to call (1) and (2) declaration-targeted
  gates and (3) a broad informational scan that will see references.

Bash checks unchanged; they already encode the declaration-vs-reference
distinction via the table-cell and bullet-prefix anchors.

.codespellrc

@@ -56,7 +56,9 @@
 
 # ans - bash and powershell variable short for answer
 
-ignore-words-list = numer,wit,aks,edn,ser,ois,gir,rouge,categor,aline,ative,afterall,deques,dateA,dateB,TE,FillIn,alle,vai,LOD,InOut,pixelX,aNULL,Wee,Sherif,queston,Vertexes,nin,FO,CAF,Parth,ans
+# GUD - "Guideline" identifier prefix in the create-implementation-plan skill spec (alongside REQ, SEC, CON, PAT, etc.)
+
+ignore-words-list = numer,wit,aks,edn,ser,ois,gir,rouge,categor,aline,ative,afterall,deques,dateA,dateB,TE,FillIn,alle,vai,LOD,InOut,pixelX,aNULL,Wee,Sherif,queston,Vertexes,nin,FO,CAF,Parth,ans,gud
 
 # Skip certain files and directories