Fix: System prompts for Guardrailing Excerpt & Title

[Takshil-Kunadia]

What?

Closes #395
Closes #399

It adds an output-format guardrail to the Excerpt Generation and Title Generation system instructions so the model returns only the excerpt or title and no conversational preamble, wrapper quotes, code fences, markdown, or trailing meta-commentary.

Why?

Both issues report the same class of bug: generated excerpts and titles arrive with extra wrapper text that gets saved verbatim into the post's excerpt / title / slug.

The root cause in both cases is that the existing system instructions do not explicitly constrain the model to return only the excerpt/title text. Frontier models mostly infer this; smaller instruction-tuned models do not, and add conversational scaffolding by default.

As discussed on #395, the agreed approach is a minimal prompt-level fix rather than output post-processing, and without bloating the instruction.

How?

Adds one requirement bullet at the end of each affected system instruction. The wording follows "eliminating preambles" recipes from major AI providers along with a positive output contract plus a short specific negative list which is the pattern empirically most robust across both frontier and smaller models.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 4.6
Used for: Researching cross-model prompt techniques for suppressing preambles (Anthropic prompt-engineering docs, OpenAI GPT-4.1 guide, Meta Llama guide). Final wording, file edits, and testing were implemented by me.

Testing Instructions

1. Check out this branch.
2. Configure a small/light model likely to exhibit the bug. Example: HuggingFace free-tier via the HuggingFace connector as described by the reported with Meta-Llama-3-8B-Instruct is what I tested with.
3. Create or open a post with ~500 words.
4. For Excerpt: in the sidebar, click Generate excerpt. Repeat 5–10 times. Expected: each generated excerpt is plain text with no Here's... / Sure, preface, no surrounding quotes, no markdown.
5. For Title: click Re-generate. Repeat 5–10 times. Expected: a plain, single-line title; no Based on the content..., no ** / ###, no (N characters) suffix, no Why this works: explanation.
6. Sanity check with a frontier model (OpenAI / Anthropic / Google) behavior should be unchanged from before this PR (the new bullet is not supposed to regress well-behaved models).

Screenshots or screencast

Before	After

Changelog Entry

Fixed - Excerpt and Title generation no longer include conversational preambles, wrapper quotes, markdown, or meta-commentary when using smaller language models.

[github-actions]

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Co-authored-by: Takshil-Kunadia <takshil@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.44%. Comparing base (219f424) to head (4a222ad).
⚠️ Report is 10 commits behind head on develop.

Additional details and impacted files

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  Coverage      68.44%   68.44%           
  Complexity       846      846           
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  Files             56       56           
  Lines           4095     4095           
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  Hits            2803     2803           
  Misses          1292     1292           

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[jeffpaul]

@sagardholakiya, @AasthaPandya, and @divyawpdev note that if you link your GitHub and WPORG profiles, then I can properly credit you in the AI plugin 0.8.0 release post as well as get the AI Contributor badge added to your WPORG profile.