Add copilot instructions for team review

[PasiVuohijoki]

Adds copilot-instructions.md into repository. This is the instruction file I've been using for a while now.

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This change is 

[github-actions]

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[jannebe]

pitäiskö hasura submodule mainita?

[PasiVuohijoki]

Lisäsin maininnan

[jannebe]

.github/copilot-instructions.md line 14 at r2 (raw file):

  - `codegen` (GraphQL code generation)
  - `test-db-manager` (test DB tooling)
  - `timetables-data-inserter` (submodule)

pois, maininta alla riittänee

[jannebe]

.github/copilot-instructions.md line 30 at r2 (raw file):

## 3) Environment and dependencies

- For local backend dependencies, use `./scripts/development.sh` commands (for example `start:deps`, `setup:env`, `setup:test`, `stop`).

tarvitseeko näitä mainita? näitä ei kai pitäisi copilotin joutua ajamaan vaan devaajan kannattanee pitää ympäristöstä huoli? tai mainita ehkä kehoittaa käyttäjää varmistamaan tarvittavien komponenttienajossa olo, jos jokin muutos sellaisia edellyttää?

[jannebe]

.github/copilot-instructions.md line 102 at r2 (raw file):

- The user always makes git commits themselves. Never commit or offer to commit on behalf of the user.

• never offer to run any git commands that modify remote repositories unless explicitly requested

[jannebe]

.github/copilot-instructions.md line 109 at r2 (raw file):

- Do not rewrite existing scripts when existing `yarn` and `development.sh` flows solve the task.
- Keep generated-file churn minimal; regenerate only when required by changes.
- When uncertain about behavior that depends on running services (Hasura, Docker stack, db seeds), state assumptions and choose the safest minimal change.

When uncertain... ...and verify assumptions from user before implementing changes.

[jannebe]

.github/copilot-instructions.md line 107 at r2 (raw file):

- Do not refactor unrelated areas.
- Do not add new frameworks, state libraries, or architectural patterns without request.
- Do not rewrite existing scripts when existing `yarn` and `development.sh` flows solve the task.

?

Suggestion:

write new

[jannebe]

pari pikku pointtia

[jannebe]

@jannebe reviewed 1 file and all commit messages.
Reviewable status: all files reviewed, 6 unresolved discussions (waiting on PasiVuohijoki).