Say that the blocks and the pin are not editable

[sehkone]

Adds a ## Shared instruction blocks section to the workflow block, and cuts 0.3.0.

The rules

Both were written only in the pull requests that deliver a release — read once, then scrolled past. AGENTS.md is the file an agent loads before touching anything, and it said neither.

• The marked regions are generated. Change the wording upstream and cut a release there; editing them in the repository that carries them is overwritten by the next delivery and turns the drift check red until it lands.
• .agent-instructions.toml is source, not tool output. Never ignore it, never untrack it — a repository whose pin cannot be read fails the check outright rather than being read as carrying no blocks.

The second is the sharper one, and it stopped being hypothetical while the seven onboarding pull requests were being rebuilt on the new path: aice-web-next and aimer-web each track 107 files under .agents/skills/, a directory they populate deliberately for agent skills. The pin had been living inside someone else's directory in two repositories, unnoticed. A generic name attracts that; a file that looks generated attracts a .gitignore entry.

Why this is 0.3.0

The entries under [Unreleased] from the pin move fold into this section. That move changed a file this repository expects to find in a consumer, which the version scheme makes MAJOR on its own — MINOR below 1.0.0.

It is also what makes the release cuttable at all. check_release_surface.sh refuses a tag whose blocks/ and repos.json match the previous release, and the pin move touched neither, so main could not be tagged before this. The block change gives 0.3.0 something a consumer actually applies.

Verification

• lint_blocks.py passes with the new section: within 76 columns, - bullets, and Shared instruction blocks collides with no heading in another block.
• All six script tests and markdownlint pass.
• blocks/workflow.md differs from 0.2.1, so the surface guard has something to compare.
• No [Unreleased] heading or link reference is left behind.

After the merge

1. git tag 0.3.0 && git push origin 0.3.0
2. The seven open onboarding pull requests get re-applied onto 0.3.0 and force-pushed; they keep their numbers.
3. The six repositories already onboarded take 0.3.0 from their scheduled apply, which also moves each off the old pin path.

Closes #42