fix(extensions): allow read-only source inspection in intake guardrail

Address the sixth review round on #3568.

The intake guardrail said the command "only reads and writes inside
.specify/assessments/<slug>/", which contradicts its documented inputs:
intake must read a codebase pointer (repository inspection) and fetch an
allowed URL to capture the idea. The guardrail now limits only *writes*
to the assessment directory and explicitly permits read-only inspection
of the supplied sources (repo + allowlisted URL fetch under the URL Trust
Policy). The other four commands already phrased this correctly ("read
only, and write inside ...") and are unchanged.

Assisted-by: GitHub Copilot (model: Claude Opus 4.8, supervised)
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 1f25cf9d-b7eb-4b2b-b811-3e91d8db8f6a

Author: mnriem

extensions/assess/commands/speckit.assess.intake.md

@@ -109,7 +109,7 @@ Record in `intake.md`: the **sanitized URL** (strip any `user:password@` userinf
 
 ## Guardrails
 
-- Never modify source files — this command only reads and writes inside `.specify/assessments/<slug>/`.
+- **Writes** are limited to `.specify/assessments/<slug>/` — never modify source files or anything outside that directory. **Reads** may include the supplied sources: you may inspect the repository (for a codebase-pointer idea) and fetch an allowed URL (under the URL Trust Policy above) read-only to capture the idea.
 - Never evaluate, size, or solutionize the idea here — that is what the later stages do.
 - Never invent origin, ownership, or context the input does not support — mark it `[NEEDS CLARIFICATION: …]`.
 - Never overwrite an existing `intake.md` without confirmation.